Bad Science

Bad Science - There are some things that persistently are taught in school even though they are not true and we have known for a very long time they are not true.

These are some of my personal pet peeves and how they actually impeded my understanding of science as a youngster. I have also included in my list a number of preposterous claims by advertisers, developers, and others.

I will entertain email offering corrections to the material here. I appreciate having my mistakes corrected. Unless the mistake is obvious, please help me by providing some detail or, better yet, a good reference. Thanks.

(In general, if the ad says the banks, manufacturers, the government, the grocers, the retailers, xxx hate this,.... then it is probably a scam. )

1. Sound does not travel in straight lines and light does

You may have seen this in a textbook. It is the kind of thing that drove Richard Feynman nearly mad when he was asked to be on a textbook commission for California schools.

The argument goes like this. There is a streetlamp around the corner. You can't see it until you move so that the corner is not along a straight line between you and the lamp. Your friend stands around the corner and talks to you. You can still hear your friend. Therefore, light travels in straight lines and sound does not.

So, even though sound is a mechanical wave and light is an electromagnetic wave, they both are subject to several rules of propagation in common. Traveling in straight lines (except for reflection, scatter, refraction, and diffraction) is one of them.

Light is also bent by gravity. This observation led Einstein to say that light travels in the shortest distance between two points (the shortest time, really - work it out with refraction and you'll see it's true) so that space must be warped in a gravitational field. That's why light seems to move in a curved path, but really it's just space that's warped. This to me is like the zero g thing. It depends on where you want to stand and make your observation. If you want to stand on the earth and look at star light as it passes near the sun, and say that the sun warped space so that the star light looks like it did not go straight, and then your law about the shortest time between two points holds. So be it. You can look at the same situation in an equivalent way without warped space and get the same answers. You just have to say that gravity attracts the light.

2. Heat can not be transmitted through a vacuum

They teach this in school. They say this is how a Thermos bottle (Dewar if you are English) works. Supposedly, heat can not travel through a vacuum and therefore the bottle is a perfect insulator. Ever wonder how the sun warms us then? Actually there are three ways that heat travels:

The vacuum bottle pretty much eliminates 1 and 2, but what about 3? Well, you could eliminate radiation if you could get your emissivity to zero. An approximation to this for room temperatures occurs with something highly reflective in the infrared, such as gold or silver or aluminum. That is why the thermos bottle usually has a silvery coating on the glass. Note that really good vacuum bottles use a sliver coating. The cheap Chinese ones usually use aluminum. They don't work as well. I tested a few different vacuum bottles here: Vacuum Bottle Tests.

3. Plants breathe in carbon dioxide and breathe out oxygen

A lot of you probably know this: plants use oxygen to breathe just as animals do. That is how they consume food and grow and so forth just like animals. So what's this about carbon dioxide? Plants make carbon dioxide just like animals. Hold on. What's going on here?

Well, in sunlight, plants which contain chlorophyll can make lots more oxygen than they use, and can consume lots more carbon dioxide than they make. This is really good or else carbon based life would be in trouble - at least the aerobic stuff. But plants kept out of sunlight go right on consuming oxygen. That is why plants are usually a bad idea in a hospital room. That is why algal blooms (sudden growths of algae) can kill all the fish - they take all the oxygen.

So if you average over all plants over all seasons, you get a net increase in oxygen and a net consumption of carbon dioxide, but remember, that is the simply story and you might get surprising results in a specific situation.

4. How a laser works

They usually tell you about how light can be amplified by a gain medium. This is a material which has been pumped into an upper energy state such that there are more molecules (or atoms or electrons as the case may be) in a upper state than in an immediately lower state. The molecules coming down to the lower state give off energy in the form of photons, adding to the light wave. Normally, this happens spontaneously, but in a laser, the timing is arranged so that a lot of it is stimulated emission. That is, a photon of the same energy comes along and coerces the molecule into releasing a photon. If there is enough gain medium, you get amplified stimulate emission, or ASE. This used to be called super fluorescence, but it was decided that this may have been misleading, so now it is called ASE. Well, everything is pretty much what they told you, but how do you get a laser?

If you add two mirrors to get the beam to trace the same path over and over in a line, you can get the light to stimulate more photons and amplify until saturation occurs. (This is a lot like a public address system where the microphone can pick up the speaker and you get a squeal. It is a positive feedback loop.) More mirrors could be used for a crooked line or a ring. The alpha laser resonator that I have analyzed over the years is quite a bit more complicated, but it is all for the same thing. To get those photons to keep going through the gain medium, sweeping up all available photons.

They told you this before, but how does the beam get out? They told you that the beam gets strong enough to break through one of the mirrors and come out. How silly! This really confuses people. Does the mirror break? Do photons suddenly stop reflecting and start going through a mirror? Of course not. One of the mirrors either has a hole in it or is partially reflective so that some light leaks through on every pass. As long as more than enough light is reflected back to keep sweeping the photons in the gain medium, the laser continues to work.

There are many videos on the subject, but I did not find many of them satisfying on this point. Here's one that I think is pretty good. Unfortunately understanding lasers in an accurate way takes more than a 2 minute video: How lasers work.

5. How Dolby works

The Dolby literature simplified the explanation for the masses. But simple explanations generally confuse.

The literature says that under conditions of loud high frequencies, the signal is recorded normally and played back normally. When the sound falls below a certain level at high frequencies, the high frequencies are boosted by 10 dB. On playback, the boosted signal is attenuated by 10 dB, and the noise is attenuated as well. All is back to normal, except that the hiss from the tape is reduced. It sounds like there is a switch that suddenly turns on a 10 dB amplifier.

On playback, how does the Dolby processor know whether the signal was boosted or not?

Well, it does not really work quite like that. The basic Dolby idea is to use a compander. A compander compresses the dynamic range (sort of like riding the gain control) during record, and undoes the gain riding on playback. The way it works is to have a continuously variable gain boost. For example, if the input is 0 dB, no gain is applied. If it is -2 dB, then 1 dB of gain is applied. If it is -10 dB, then 5 dB of gain is applied. All input levels are mapped to new output levels. Now if your signal on playback is -5 dB, you know that it was originally -10 dB, so 5 dB of attenuation is applied. If you encounter -30 dB, then you attenuate 30 dB. It is a continuously variable gain function, not something that switches at 1 dB levels. dbx made such a compander for professional and home use (but it worked across the entire audio band, not just high frequencies.)

Dolby A was set up for professional use. It divided all of the sound into a dozen or so frequency bands and processed each one through a compander. It worked so well, that a cheaper, simpler version was set up for consumer use. Here, only one band would be companded - the high frequencies. The lower frequencies were left untouched. The consumer version was called Dolby B. A somewhat refined version that played with the tape bias level allowed for some additional headroom on tape - something greatly needed by cassettes. This was called Dolby C.

This particular tidbit of information is obsolete. Dolby is concentrating on digital sound now. Anyone under thirty probably thinks this subject is ridiculous. If you happen to have some old Dolby cassettes and want to play them back on a cassette deck without Dolby, you may find it difficult to get things set up right. On the other hand, the vast majority of tapes and players were poorly set up anyway. You can get good results simply by rolling off the high frequencies by 5-8 dB, or you can do better by setting up Audition or some other software to expand the dynamic range of frequencies above 2 kHz. It won't be perfect, but with an old cassette perfection is not going to be much of an option anyway.

6. Fake experiment shows oxygen consumed by fire

You may have seen this: a candle is lit and placed in a pool of water. A glass is inverted and placed over the candle, which continues to burn for a while, and finally goes out. Behold, the level of the water has risen in the glass. You are told that the water replaced the oxygen that was consumed.

Can you figure out what is wrong? Now think about it a moment. The oxygen was consumed? It disappeared? Of course not. Each molecule of oxygen consumed is converted to either two molecules of water vapor or one molecule of carbon dioxide. You probably remember that to a good approximation all gas molecules occupy the same space at room temperature and pressure. This says that probably there should have been too much gas under the glass and it should bubble out, not pull water in. What gives?

As the air is heated, it quickly expands. It continues expanding, even after the glass is brought down over it. Some of the expanded gas escapes unnoticed as bubbles from the glass. When the candle goes out, the gas rapidly cools and takes up less space, pulling up water as it condenses.

7. A Radiometer demonstrates the principle of photon pressure

8. Is Alternating Current deadly?

This is what was claimed by company interests as Edison went against Tesla. The direct current users and sellers did not want to change, because of investment in DC equipment that had already been made, so the first thing they thought of was that AC current might be more dangerous than DC. However, AC had the advantage of being easily stepped up to high voltage for transmission through long distances, like the next town over. It could then be easily stepped down for home use near the home.

That AC was inherently more deadly than DC was asserted in trying to prevent switching over, but it did not work. A lot of people were genuinely afraid of alternating current. One thing that might be said is that because of the body's capacitance, alternating current can cause a tingling sensation at much lower voltages than direct current can. Some people can sense relatively low levels of AC current, but this does not in itself create more danger. .

9. Centrifugal Force

But there is no real centrifugal force. If there were, then it would balance the centripetal force and you would fly off in a straight line instead of going around in a circle.

If you are inside a box mounted on a rotating arm, and you ignore what is really going on, and pretend that the box is not moving, you then "observe" a force pushing you toward the outside edge of the box, that is the edge further from the center of rotation. You look at this situation from the outside, you can see that your inertia is resisting being forced to go in a spinning circle. In actuality, the box is pushing on you, forcing you toward the center of rotation. But when you are sitting in the box, it feels like some mysterious force pushing you toward the outside edge of the box. Try releasing a stone while you are in this spinning machine. From your point of view it takes off in a strange looping path. But seen from someone outside your box, not spinning, the stone simply goes off in a straight line.

If there were a real force pushing you out and balancing the force pushing you in, you would feel no net force and you would move in a straight line at a constant velocity. It is the unbalanced centripetal force that makes you move in a circle.

10. Zero G in Orbit

What do people think of when they hear the term 0g? At first you might think that something is not accelerating. Next, you may remember that standing on earth you are not accelerating, but you experience 1g. So you then think, well, in orbit, you are so far from the earth that gravity is negligible, so that's it. This is not true, gravity is still alive and well, at about 97.5% of the value at the earth's surface. How can this be?

So actually, you weigh almost the same, and you are accelerated toward the earth by almost the same force, but you don't feel it. Why? Because the ground is not there pushing back on you to stop your acceleration. Anything in orbit is actually in free fall. If you jump off a building, while you are falling, are you in zero G? Well, in the way that we mean 0g and microgravity, yes, you are "in 0g" in free fall. Only in that sense you are falling, blood does not pool in your legs and if you release a ball, it falls right with you. So if you allow yourself to be a (moving) reference frame, you are in zero G. (However you are in a non-Newtonian reference frame.)

Now suppose you are being forced to go in a circular motion around a planet by gravity. Gravity is constantly pulling you into a circle rather than allowing you to fly off in a straight line. Are you weightless? No, you are still being pulled toward the planet. That is why you do not fly off in a straight line. Everything around you, your spacecraft, your clothes, your hair, your glass of water, is moving in the same circular motion, so if you forget about the motion for a moment, everything seems to be standing still. If you let go of something, it does not change motion, so it seems like there is no gravity. But that's only because you are not used to falling. You are used to resisting gravity by pushing on the ground to stay up.

But it is not truly a 0 g environment. Calling it that is just as fictitious as "centrifugal force", but people still use the term. (What other term would you use to describe the floating sensation?) The only way you would actually truly experience zero G in an inertial reference frame would be if you were far from any large mass (planets, stars, etc.) Then you would not be moving and not experience any relative acceleration between you and objects around you. It would seem pretty much the same as being in orbit, except you would not be going around anything. Now Einstein reasoned, if there were some large gravitational acceleration in some direction, that affected everything I could see, and I were free falling in that gravitational field how would I be able to tell? Since velocity was relative, maybe acceleration was, too. However, if light were not affected by gravity, you'd be able to measure the acceleration relative to light and deduce the falling. Einstein made a leap of faith to say, I believe that acceleration is like velocity, and you wouldn't be able to tell. He decided that it was like compressing space, so everything thing in that space would be affected the same way, you could not tell unless you were outside that space looking in. In that way, he predicted the effect of gravity on light. This was astounding, since light has no mass, and was believed not to be affected by gravity. Experiments prove that light is definitely affected by gravity, just as Einstein reasoned it would.

Now the problem with using terms like "weightless", "zero-G", and micro-gravity is that you have to understand that no one really literally means these things in the normal, Newtonian view of the world. To show how seriously confused people get by all this, here are a couple of letters to a well respected aviation magazine from reasonably intelligent readers:

I don't know about you, but I quickly scanned these two letters and thought these guys were really mixed up. However, if you accept a reference frame flying along with the vehicle, they are actually not too far from the truth. Let's take their statements one at a time:

You might look up "weightlessness" on Wikipedia for more information. The last time I checked, this was a fairly accurate article. I have gained a lot of respect for Wikipedia, since misconceptions are usually corrected by someone who knows better in a short length of time.

11. Retro Firing Slows Down a Spacecraft in Orbit

This is of course, counter-intuitive. But air drag and retro firing are only minor forces acting on the spacecraft.

Air drag, or retro firing removes orbital energy from a satellite. This forces it to fall closer to the earth. But at its new altitude, however, the orbital velocity is higher due to stronger gravitational acceleration and to a smaller orbital radius. Some of the potential energy has been converted to kinetic. Thus, firing a retro momentarily slows down the spacecraft, but ultimately speeds it up as it falls closer to earth.

12. Curved wing supposedly creates lift by Bernoulli effect

They told you an airplane is lifted by lower pressure on top the wing than below. Let's call this Bernoulli lift. Would you believe that much of the lift is created by pushing downward against air? This of course sends the air downward. Let's call this reaction lift.

Try a simple calculation of the Bernoulli effect: take a 2000 lb plane with a wing area of 20 sq feet. Assuming that the air could be made to stick to the wing and move 10% faster over the top than the bottom, you might see a 1% change in air pressure from top to bottom, resulting in perhaps of the order of 0.15 lbs per sq inch. The total lift is of the order of 500 lbs. That is not enough to lift a 2000 lbs. plane. Clearly, the airplane needs more lift than just this (it manages to fly, does it not?). The extra lift comes from throwing some of the air downward to create an equal and opposite force pushing the wing up.

On a typical airliner, you might experience 15-20% of the total lift from Bernoulli effect, but 80-85% of the total lift comes from reaction lift. Ever see a stunt plane fly upside down? How would that be possible if the Bernoulli lift were dominant. Wouldn't flying upside down create a downward force in addition to gravity?

Look again at that jetliner before you board. The wings are not held level, but they stick up in the front and down in the back. That way, they are constantly hitting air and forcing it down, creating lift. Furthermore, the plane will often fly in a slightly nose up attitude to create more lift. There are limits to this, however. When the stream of air breaks up from laminar flow over the wing to turbulent flow, your nose is too high and you are going to stall.

Years after writing the above, Airbus sponsored a minute physics video explaining this. I hope you enjoy the video here: How do airplanes fly?

13. Diode rectifies because of holes and electrons

The standard electronics books and classroom textbooks gave a totally nonsense explanation of how a diode works. The explanation usually goes like this: in the "N" material, conduction usually occurs by the movement of electrons. Electrons flow opposite to the direction of current. (Current flow is defined arbitrarily to be in the direction of positive charge flow.) In "P" material, there is an excess of "holes." (Holes are where an electron should be to fill up an orbital shell, but there is one missing because of the P type doping. So in P material, conduction occurs by the flow of "holes" in the direction of the current. Since holes and electrons flow in opposite directions, at the P-N junction holes and electrons either have to come together (allowed) or flow apart (not allowed.) Some kind of gibberish about holes and electrons can annihilate each other, but holes and electrons can not be created out of nothing is offered. This all falls apart because holes are just a fictitious invention like centrifugal force and coriolis force. Besides, why can electrons and holes flow out of a metal junction with P material?

OK, so what really happens? At a P-N junction, the dopants cause excess charges to be built up at the interface. These charges are held in place by the nature of the materials, on average, so that there is an average permanent electric field in the semiconductor. That mean that for any applied voltage in the same direction as the field, current will flow. For voltages in the opposite direction that are smaller than the permanent field, current will not flow. If a voltage is applied that exceeds this permanent field, it is called "break down" and current does flow, with likely damage to the device.

14. Atoms are composed of little balls

It is sometimes tempting to describe atoms with electrons and protons and neutrons like miniature solar systems with particles orbiting around others.

There are many problems with thinking of atoms in this way. They simply do not behave like miniature solar systems would. For example, only discrete periods and radii are allowed. And the orbits do not decay by radiating electrical energy into space as you would expect. And you can not predict when and where the electrons will be in the way astronomers predict the configuration of planets.

On top of that, the electron motion is quite complicated for most atoms. The outer electrons act as if they are moving in a very irregular orbit with lots of loops. And if the electrons are fermions (they are) they interfere with each other in ways that planets never do. On the other hand boson particles do not do this.

It has been discovered that a wave function can be defined for each particle such that the square of the wave amplitude represents that the probability that the particle can be found at a position. Wow, that sounds useless or weird or something out of this world. What do you mean the probability that a particle can be found at a particular position. Where is it? Well, the weird thing is, you can't tell very accurately until after you stop it or change it in some way. Then you will no where it was when you changed it, but you can't tell accurately just how you changed it! OK, this is really weird right. You mean you don't have instruments good enough, right? No, I mean it is a fundamental property that you can't tell where it is. You can't confine it to a smaller volume than a certain amount, because its position and velocity combination is always a minimum size. This is what keeps matter from collapsing under the weight of gravity. This is just the start of the weirdness. By thinking of the atoms as little planetary systems, you predict everything incorrectly about the way they behave.

15. Evolution as taught by Darwin explains the diversity of life forms and the complexity of higher life forms

Evolution is supposed to happen by normal variations in species specimens that have a differential advantage in breeding. The most common advantage is survival to breeding age. Somehow this single factor is supposed to account for the whole process.

This explanation for evolution was arrived at by watching domestic animals bred in captivity -- noticeable changes are done in a few generations by selecting for particular characteristics. Couldn't nature perform a selection process based on competition for survival? Of, course. And with millions of generations to work with, couldn't even the smallest competitive advantages be enhanced?

Well, I think the answer is yes and no. The presumption is that the selected characteristics are passed on to the offspring. But consider apple trees. Such a process could never be used to select trees for good tasting apples. The fact is, the seeds from apples when planted only have a 1 in 800 or so chance of producing good tasting apples. Even if the seeds came from inside a good tasting apple. (Most apple bearing trees that produce edible fruit are grafted.)

Ever ask why there still exist so many species when supposedly evolution would have made something better at surviving by now. Whatever was better should have left crocodiles, wolves, elephants, and monkeys in the dust bin by now. And there are not too many intermediate things around, you know, half way between species. Its as if genetics do not really allow an infinite range of variety of species, but maybe there are quantum jumps. Hmm, though, now this is interesting. Evolution as explained by Darwin does not take these ideas into account.

Is it possible that due to imperfections in the genetic copying process that you get an offspring radically different from the parents? Well, yes. Most of the time, we consider these to be birth defects. But once in a while, the changed gene, not inherited from any parents, but an accident is beneficial. It may be something hardly noticeable like a natural immunity to AIDS. (Notice that word, "natural"?) Other times, it might mean a salamander without legs or a snail without a shell, or naked monkey with a big head.

OK, well if a birth defect like this occurs, how would it make a new species -- how would it find a mate? Interesting question, and there are some obvious directions for speculation here.

The fossil record seems to show quantum jumps in species, not a record of continuous change with lots of individuals intermediate between two species. The fossil record seems to indicate quasi-stable species designs which do not vary over long periods of time.

Other factors that may be involved are: population isolation for large periods of time, point mutations, cataclysmic environmental changes on a relatively frequent basis....

It would seem that this whole subject is vastly more complex than the simple rules that you might have been taught like "survival of the fittest." Other factors often outweigh survival of the fittest.

16. Why is the sky blue?

Why is the ocean blue?

Why are glaciers blue?

Is the sun yellow?

The questions were grouped together because we first have to deal with a usually unspoken misconception about color. To illustrate what I mean, can you describe the difference in color between white and silver or between gray and silver? How about the difference in color between yellow and gold?

If you have ever struggled with that, you may realize that the “color” of silver and white is actually the same. What’s different is the directionality of the reflection. Optical scientists usually characterize the difference as something called a Bidirectional Reflectance Distribution Function (BRDF). This differentiates a white sheet of paper --where the light is diffusely reflected in all directions-- from a polished silver surface in which light is reflected out in only one direction for each incoming direction. This type of reflection is called a specular reflection because it is the way a mirror reflects.

We usually think of objects as being one of these two types of reflecting bodies. In fact, we try to think of and speak of most objects as if they were diffuse reflectors. Many objects more or less fit this model.

However, we soon notice that many objects have combinations that words do not describe well. A “red” coffee mug might be ceramic with a clear glaze. So the glassy coating may specularly reflect all colors of light, while the diffuse surface underneath may reflect mostly red light. So what color is it? We usually just say “red”. But it is not red like a “flat” red painted surface is red. So we might call it a glossy red, but again if we look closely, the glossy part is more like white than it is red.

If we look really closely at a lot of objects that we call “white”, we find that they actually have a bit of depth to them, like milk. Light does not just reflect from the upper surface, but it penetrates a bit. White is usually made up of very small clear particles like a cloud. Light refracts from all these particles and eventually leaves through the same surface that it entered. From a distance, it’s just “white.” But really close up, it’s a milky layer.

Suppose we took all those clear particles and melted them together into one single layer. Now instead of “white” we might call it “colorless.” We mislead ourselves with such language. Think of ordinary snow. It is composed of a very large number of clear crystals. All of those surfaces do so much refracting and reflecting of light that it can be considered a random scattering volume. We like to think of it as a white surface, but that is not really so. Light scatters down some depth into the snow. Think of this: when the snow melts into liquid water, how does it change from white to colorless. Where did all the white go?

If we had many small clear particles with blue dye in them, the substance would appear flat blue from a distance. If we melted them together, we’d have a transparent blue glassy substance. Yet, we still call both objects blue. So, obviously, we mean different things by calling objects “blue.”

Now take ordinary window glass. We tend to call it colorless or clear or transparent. However, when we look through sufficient thickness of the glass we notice that it is greenish.

Water is a little like the glass, only instead of greenish, it is bluish. It is not as strongly bluish as glass is greenish, so it takes more depth of water to appear noticeably blue.

So, ocean water is bluish, but a transparent blue, not like flat blue wall paint, or even glossy blue, but more like a cobalt blue glass mug.

The ocean is blue for two reasons. If you look through it to the bottom, it is blue because of the absorption of red light. If you look at the sky reflected from the surface of the ocean, you are seeing blue because the sky is blue. It’s blue because the water weakly absorbs reddish light. A good paper that describes the phenomena is Modeling the reflectance spectra of topical coastal waters by several faculty and staff members of the National University of Singapore. They include a model for the absorption coefficient of seawater. You can also get absorption coefficient data here.

Now, what about the sky. The sky is blue sort of in the way that snow is white. It is due to scatter. I have seen websites where it is claimed that the air is a bluish substance, and that would be the reason why. This is incorrect. If the atmosphere were completely transparent without any scatter, you would see a black sky, same as on the moon. If the air were a bluish substance, you would still see a black sky, but the stars and the sun and the moon would appear blue. In fact, most everything would appear blue because only blue light would filter down from the sun. Obviously, this in not what is happening.

The only way the sky can not be black is if the air is scattering light from the sun into our eyes no matter what direction we look. The light scatters from aerosols, particles of dust, and from the air molecules themselves. It turns out that unless there are volcanic remnants, clouds, or smog, that the scatter from the air molecules is the most important scatter. Some people have claimed with good reason that scatter from man made particulates has made the skies brighter than they were before the industrial revolution. At any rate, scatter from molecules is very strongly wavelength dependent. This type of scattering is called Raleigh scattering after the first person to describe it fairly accurately. The actual dependence on wavelength is λ^-4 meaning that as the wavelength goes from red (600 nm) to blue (400 nm) the strength of the scatter increases 5 fold. So blue light scatters 5 times as strongly as red light. This applies mostly to light that is scattered at large angles from the sun. There is also a strong polarization component to the scatter which you can observe using polarizing filters, but that is really beyond the scope of understanding why the sky is blue. This dependence of the scatter on wavelength can easily be seen in a spectral plot of the skylight.

What about glacier ice and snow? We already know now that water is blue, and we now learn that ice is even more blue. Hey wait a minute, isn’t is colorless or white? Actually no. It may seem white in a skating rink, but only because the trapped air in the ice does not let light penetrate very deeply. Glacier ice is more densely packed so that the air bubbles are far fewer and farther between than in ordinary ice. The result is that light penetrates far deeper. Now the light has to traverse much more ice before it reflects or is scattered back This gives a chance for much more red to be absorbed along the path, hence the ice appears much more blue than in “normal” ice or snow. If you visit Juneau, everyone will tell you “the snow is not blue, it’s just an optical illusion.” What they mean is that the glacier is backlit so that you see sun light not bouncing off the top layer of snow, but sunlight filtering through a lot of snow and ice. When that happens, the weak red absorption in the ice plays an important role in what you see.

17. Why Are There Tides?

Figure 17-1

Note: moon shown much closer than actual scale. Tidal bulges have been greatly exaggerated so that you can see them. The real tidal bulge is less than one millionth of a pixel on this scale. Moon would be more than three feet a way from the earth on this scale.

This subject is so extensively treated in textbooks and on webpages authored by university physicists and astrophysicists that you would think they would have got it right by now.

Unfortunately, the subject is so full of half truth, incomplete information, and serious errors that someone who thinks about what is going on is almost sure to be confused. Also, unfortunately, to truly understand the cause of the tidal bulge on both sides of the earth, you probably need to be a good physics student. You will need to know about non-inertial reference frames and weightlessness in free fall. To actually calculate the amount of tidal bulge, you need to be able to formulate the gravitational potential field and the centrifugal force field potential. It turns out to be a bit complicated for an oblate earth, but we can get a good approximation for a spherical earth. I will try to guide you a bit, but these concepts usually do not come easily the first time, so if I lose you, don't give up. Read more.

We will begin for the moment by asserting without proof for the moment that the largest contributor to tides is the gravitational pull of the moon. In doing this we have to realize that the gravitational pull of the sun is much greater than the pull of the moon, and that the rotational effect of the earth does in fact result in a bulging out at the equator. The sun's effect is less than that of the moon for reasons that we will see shortly. The equatorial bulge is almost exactly constant, so it does not cause a daily movement of the water level. Therefore, we can accept that bulge and ignore it.

Let's start by dismissing the school textbook non-sense about the earth rotation centrifugal force as the reason for the tidal bulge on the opposite side of the earth from the moon. If your common sense does not tell you that the centrifugal force is constant, then maybe you should not be reading this. Since the centrifugal force also acts on the earth as a whole, over time the land mass and core of the earth have flattened, so the water is not deeper at the equator

This diagram is also somewhat misleading because the moon is shown in line with the earth's equator. It actually on average is tilted with respect to the equator.

Figure 17-2

Typical school textbook shows a lot of very misleading and some downright wrong information. Typically no mention is made of the fact the drawing scales are wrong and that centrifugal force is “fictitious” and that it causes a bulge around the entire equator, not just one side. No moving tide results from centrifugal force as shown. However, in a very non-obvious way, it is due to centrifugal force of the earth's revolution (orbit) about the moon! This is probably not the best way to describe the situation or understand it so I won't describe it this way. See text.

To be complete, we have to admit here that centrifugal force is not a real force in a physicist’s point of view. There is nothing that causes centrifugal force, because, as I said, it is not a force. Rather it is an imagined force that we have to add to make Newton’s laws of motion work if we insist on standing on a rotating earth and pretending that we are standing still and the laws of motion still hold. This is as ridiculous as being in a car falling off a cliff and saying that there is a force that balances gravity that keeps objects in the car from being attracted to the earth. Everything in the car is in “zero g” or “weightless” is it not? Well, if you say “no, everything is falling at the same rate as the car” you are right. But if you insist in pulling down the shades and pretending your car is motionless, you have to have “zero g” to explain why things are floating (tossing maybe) around in the car.

Figure 17-3

Illustration of the moon's orbit being inclined relative to earth's axis of rotation. The moon is shown too small and too close to the earth relative to the actual scale. Tidal bulge is also enormously exaggerated so that it can be seen.

As earth rotates under tidal bulge, tide appears to go out and come in. Near the equator, the bulge is not large compared to the unperturbed state of the oceans, so tide appears to ebb and flow much less than at mid-latitudes.

So exactly the same is true for centrifugal force, coriolis force, and “weightlessness” in orbit. These are not real physical laws, but they are used for the convenience of working in a reference frame that is either spinning or accelerating. The more accurate description of the situation is that you are choosing to work in a non-inertial reference frame. In this type of reference frame, a force is actually required to accelerate the objects or to force them into a spinning motion, since all objects will in the absence of any force either remain motionless or continue to move with the same velocity. Since velocity means speed in a particular direction, this is equivalent to saying that the object continues to move along a straight line at a constant rate.

Now it is absolutely necessary to clearly understand all of this to explain the tidal bulges on the earth due to the moon. Some astrophysicists have published web pages and textbooks which say otherwise. The worst of these claim that the tidal bulges would still be the same if the earth and moon were held in place. The claim is that the differential force between the moon pulling on the center of the earth and the ocean is what cause the ocean to take its shape relative to the earth.

Actually, if the earth were held in place, then whatever were holding it in place would be canceling the gravitational force of the moon. The force acting on the ocean would be the vector sum of the force from the earth and from the moon. In this case, there would be a strong bulge of the ocean on the side of the earth toward the moon, and a deep depression on the side away from the moon. Your intuition probably told you that.

Note: I drew this lopsided bulge of the ocean without thinking very hard about it. After I calculated the equipotentional surface, I found a different shape for the ocean under these circumstances. Do you know what shape it would be? (See the top two drawings in Figure 17-8.)

Figure 17-4

If earth and moon were held motionless in space, the tidal bulge would be enormous and lopsided to one side. Note: not drawn to scale. Bulge shown much larger than would be to scale and earth shown much closer to moon.

Figure 17-5 Photo of the earth and moon showing the scale of the distance between the two.

So now, let's release the earth and let it start falling toward the moon. (The moon will start falling toward the earth as well.) So doesn't the ocean become “weightless” as the earth falls toward the moon? Well, sort of. In the moving reference frame of the earth, the ocean and the earth are falling at nearly the same rate, so the ocean is nearly weightless with respect to the moon. Of course, the gravity of the earth still pulls on the ocean. In fact the moon and the earth both still pull on the ocean so that the total force on the water molecules is the vector sum of the two forces. But if you insist on standing on the earth and ignoring the fact that you are falling, you would say that some mysterious force has cancelled the pull of the moon.

But notice that the earth is large enough that the pull of the moon's gravity varies significantly from one side of the earth to the other. The water molecule on the near side of the earth to the moon is attracted more strongly than the earth as a whole. It is trying to fall toward the moon faster than the earth is. Of course, the earth's overwhelming gravity keeps the water molecule from flying off toward the moon, but it does manage to be a little farther from the center of the earth than it would be without the moon's gravity.

What about the opposite side of the earth? Isn't it true that the moon's gravity pulls on water molecules there, too. Yes, of course, but since the earth is in free fall, the moon is actually pulling more at the center of the earth than on the water molecules on the far side. The earth is falling toward the moon more than the water molecules on the far side. Of course, again, the overwhelming gravity of the earth keeps the water molecules from being left completely behind, but they do lag a bit, enough to raise an ocean tide.

Now, you may be wanting to say, “that's OK for an earth that is falling toward the moon, but we are staying the same distance away, so what's happening.” Well, this might be surprising, but the earth is falling toward the moon, or more precisely in a free fall. It turns out that the earth actually orbits around the center of mass of the earth-moon system. This center of mass is actually some 1710 km below the earth's surface, so most of the time, we tend to ignore this orbital motion as being very slight compared to the orbital motion around the sun.

If you still want to say, but how is the earth “falling toward the moon” when the distance remains constant, you should read the article on “weightlessness in orbit” before continuing. If you have not mastered an understanding of that, the earth-moon orbital motion will not make the slightest bit of sense to you.

If you have followed the discussion thus far, you will understand that in free fall, because the reference frame you are using is a non inertial reference frame like the earth in orbit around the barycenter of the earth-moon, you can use a fictitious force or set of forces to describe the motion of the water molecules in the ocean around the earth. Again, this is analogous to the coriolis force and the centrifugal forces. It is an invention to explain the motion that you would observe in a non-inertial reference frame. If we use this invention, we can ignore the general average pull of the moon on all objects on the earth. Since we are pretending that the earth is standing still, we have to have a fictitious force that cancels that average pull of the moon. Once that is done you are left with the differential force of the moon's gravity relative to its average (or the pull at the center of the earth.) The differential field is also called the gradient, because it is a vector differential. However, it is not essential to know that anything about gradients to understand the tide. What is important to understand is that the differential pull of the moon is what counts because the earth is in free fall. The instant you constrain the earth (ignoring inertia for the moment) the full force of the moon comes into play, because you have stopped your reference frame from accelerating, and you are working with the real forces, not the fictitious ones that apply in a non-inertial reference frame. With the full gravitational force of the moon, the oceans would be attracted moonward into a lopsided bulge toward the moon.

Figure 17-6

Differential forces acting on earth and oceans. After the overall large gravitational field of the acting mass (moon) is subtracted out, the remaining field is shown. The reason that the overall average field can be subtracted out is that the earth as a whole is in free fall toward (orbiting) the center of mass between the earth and the moon.

Something subtle is going on here. Because the earth is not spherical, the gravitational force is not radially symmetric. If you subtract out a radially symmetric force, the remaining residual is as shown. The effect of the non spherical earth and the centrifugal force are of the same order of magnitude.

Going back to the free fall reference frame, we can draw the apparent or effective forces on the water molecules as in the diagram. Please note that the tidal bulge is greatly exaggerated so that it is noticeable to the eye and that the moon is shown much closer to the earth than it should be in a scale drawing. This was done to prevent the moon from being off the drawing or the drawing having the earth so small you could not see any detail. This force diagram is essentially the same as you will see on several webpages and in a few textbooks. What they fail to explain is why the differential forces are what actually apply in this non-inertial reference frame.

Sometimes (usually in fact) it is asserted that the differential forces would behave that way even if the earth and moon were being held still. I hope by now you would agree that that would be like saying that centrifugal forces and coriolis forces would still be there if nothing were spinning.

Something like this can usually be understood different ways. It all depends on what reference frame you want to choose to work out the physics in. If you choose an inertial reference frame, and as long as all velocities are small compared to the speed of light, and the gravitational fields are not too large, Newton's laws of motion will be followed.

Consider for a moment a large hollow space station. Let's put a little water near the center of this station. Let's assume that this space station rotates so that the same side faces earth all the time. Let's call this the down side. Over time, you will notice that if everything is very quiet aboard this space station, water will tend to collect on the down side and upside of the space station. In other words, there will be a very slight “tide” on the station. If the station rotates at some other rate, this tide will roll across the space station, much as the tide rolls across the earth. How does this work? The water on the down side, being closer to earth. It is pulled by earth's gravity a little harder than the space station as a whole. It must either orbit faster or fall a little bit toward the earth. Once it makes contact with the space station surface, the intermolecular forces make it loosely a part of the space station and it can not get any closer to the earth. Water on the upside is pulled a little bit less strongly by the earth, and is going slightly too fast in orbit, so it does not follow quit as tight a circular path around the earth. From inside the space station, it appears to move up away from the earth until it touches the side of the space station.

Figure 17-7

Illustration of tidal effect is due to gravity gradient across the dimensions of a large orbiting body.

Figure 17-8

In orbit, the gravity gradient across the size of the space station will cause an apparent force to push the water away from the center and towards the earth and opposite earth (down and up) sides of the space station.

The space station shown here is extremely large – a couple of thousand miles across – compared to any man made object ever made.

Apparent forces are fictitious, because the forces only exist if you sit in the space station and pretend that you are not moving around the earth. True force is the gravitational force of the earth which always pulls toward the earth, never away.

If earth and spacestation are held apart by some fixture, and the earth is not rotating, then all the water falls to the earth side of the space station. Gravity does reach out into space.

Suppose you stopped the space station and held it up at a high altitude. Would the tide inside the station continue unchanged? You intuition should tell you that all the water would fall pretty strongly toward the down side of the station. You would be right. Can you think of how this applies to the earth-moon system and tides on the earth? Do you see why the tides would be very different (i.e. one huge bulge on one side of the earth) if the earth and moon were held still in space?

So why is the explanation for the tide taught in a false manner? They reasons for this are complex. In the pre-college text, it is a required subject and therefore must be taught. To teach science without it would be incomplete. Does the high school teacher understand the tides? Usually, no. If the textbook went into the explanation above, not even the teacher could follow it in most cases. So what is the goal of the education? Well, ideally that you should be taught correct science. But that goal really is practically unobtainable. So, the compromise is to teach you something that might be plausible and maybe give you an inkling of the true reasons. Well, why does the astrophysicist teach it incorrectly? It is easy to get confused. Although the earth is an accelerating, non-inertial reference frame, in that it revolves around the sun, the moon (actually the centers of mass between the earth and moon and earth and sun) and rotates on its axis, and revolves around the barycenter with every other planet, we tend to ignore that for everyday things. In our discussion, for example, we conveniently ignored the free fall of the earth, oceans, and moon toward the sun. By free fall, I hope you now immediately think of orbiting the sun. In fact, all of that free fall that we ignored is not exactly constant. When the moon is closer to the sun that the earth is, it is pulled more, and when it is further from the sun than the earth is, it is pulled less by the sun. That causes the moon's orbit to bulge slightly in both directions. It also causes the oceans to bulge toward and away from the sun.

The sun's gravitational pull is much greater than the moon's, but because we are in a non inertial reference frame orbiting the sun, the differential gravity field of the sun applies. That is why both the sun and the moon have significant tidal effects. The sun's effect is on average about 46% of the moon's effect, even though the gravitational force of the sun on the earth is much, much larger than the gravitational force of the moon on the earth.

Hopefully, it will have been obvious, that the reason the tide raises and lowers twice a day is that the earth is rotating under the tidal bulges.

I set about to calculate the equipotential surface of the ocean on the earth using the following assumptions:

The earth is a sphere of uniformly distributed mass centered at 0,0,0.
The sun does not move as the earth orbits it, and may be found at -d_s,0,0.
The earth orbits the moon in a small circle with a radius of about 4500 km. The moon is at a distance of dm, and the earth's orbital size "around the moon" is d_me.
The sun and the moon are above the earth's equator.

The gravitational potential energy is the potential energy that an object of a given mass has by virtue of its position with respect to the earth’s center of gravity. If the earth were a point mass, the force on the object would vary with position according to

Where F is the force of gravity, m_e is the mass of the earth, G is the gravitational constant, m₂is the mass of the object in question, and r is the distance from the center of gravity. The work done on the object in moving it to the position r is given by

However, since F varies with r, we have to do an integral:

This integral is easily evaluated and is found to be

Usually we define a potential that is independent of the mass of the object. This potential is usually represented by the symbol U:

If we place our origin at the center of the earth, we can use Cartesian coordinates to write

The reason for choosing Cartesian coordinates will become clear as we look at the potential when we add the sun, the moon, and centrifugal force into the picture.

For example, to add the sun, we need two terms for the force. Let’s define r_s as the distance from the center of the sun. We will also talk about the gravitational field rather than the force on an object of mass m₂. The gravitational field due to the sun then is given by

from which it should be clear that the potential is given by

If we place the x axis so that it goes through the center of the sun, the sun will be at (-d_s,0,0) in Cartesian coordinates. Therefore, we can write the potential as

If we wish to calculate the potential due to the centrifugal force of the earth going around the sun, the field is:

By integration as before, the potential is given by:

With these formulas for potential, we can now find equipotential surfaces.

Based on the formulas in the above box,

the gravitational potential due to the earth is given by

The gravitational potential due to the sun is

The gravitational potential due to the moon is U₃= -Gm_m/sqrt((x+dm)²+y²+z²)

The centrifugal potential due to the earth orbiting the sun is U₄=-ω²(((x+ds)²+y²))/2 where ω is the angular velocity of the earth about the sun (2π/year). Note that there is no dependence on z, which is parallel to the axis of the earth's orbit.

The centrifugal potential due to the earth orbiting the moon is U₅=-ω²(((x+dme)²+y²))/2 where ω is the angular velocity of the earth about the moon (2π/month) . Note that there is no dependence on z, which is parallel to the axis of the earth's orbit.

This following chart gives the result under various conditions.

Figure 17-9

Size of tide/oblateness not to scale

See table below for explanation

Figure 17-9 A Top left corner. Earth on a stick and tide caused by sun. Surface determined by U₁ and U₂. Did you guess that it would be an offset circle? The tide would be 7 km high and go around the earth once a day. Off course in reality, there is so much water that it could not actually move around the earth once a day. Sun located off to the left.	Figure 17-9 B Top right. Earth on a stick and tide caused by moon. Surface determined by U₁ and U₃. Did you guess that it would be an offset circle? The tide would be 43m high and go around the earth once a day. Moon is located off to the left.
Figure 17-9 C Left center: Earth in orbit and tide caused by sun. Surface determined by U₁, U₂, and U₄. Did you guess that it would be an ellipse? The tide would be 24.6cm high and appear around the earth twice a day.	Figure 17-9 D Right center: Earth in orbit and tide caused by moon. Surface determined by U₁, U₃, and U₅. The tide would be 54.3cm high and appear around the earth twice a day.
Figure 17-9 E Left bottom: Earth in orbit and oblateness caused by sun. Surface determined by U₁, U₂, and U₄. Here instead of looking at the earth down from the north pole, we are looking at the equator. This shape contributes to the overall oblateness of the earth, but does not cause a tide, because it is orthogonal to the earth's rotation.	Figure 17-9 F Right bottom: Earth in orbit and oblateness caused by moon. Surface determined by U₁, U₃, and U₅. The moon contributes about 15 meters in radius to the overall oblateness of the earth.

I am willing to share the MatLab program I wrote to plot the above figure if you are interested.

More interesting information can be found by looking up "tide" on Wikipedia. One of the interesting things is that since tides are not steady state with the earth simply rotating under them, the waves can be funneled into very large effects where the wave moves up an ever narrowing and shallowing channel. This is why the tides can cause enormous flows in the Bay of Fundy.

Here are some miscellaneous items that I put in a different category: they are not almost universally taught incorrectly, but they somehow seem to be common misconceptions that have developed, probably partly due to newsprint, television, and the movies. Some of it is due to brevity of speech. Rather than give a long explanation and be exact, hopefully most of us scientists have taken to a more layman explanation in everyday life just to keep people from running away from us and going insane. Here are some of the ones that come to mind:

18. Does glass flow? Is glass a liquid?

This is a subject taught either in middle school or high school, although it is not at all clear why. It is at the very least misguided and premature. At worst, it is completely false and leads to all sorts of confusion.

The accompanying figure explores what is normally taught about glass flow. Glass is an amorphous solid, they say. The very word implies that glass will flow under normal room temperature conditions.

The truth of the situation is that unlike many solids, glass does not solidify at some well defined temperature. It becomes thicker and thicker (more viscous) as it cools, leading to the mistaken impression that it is still flowing at a measurable rate all the way down to room temperature.

In fact, by the time glass reaches room temperature, it does not flow any more than a solid, and that is at a rate that is too small to be ever observed.

If glass did appreciably flow, this would be a major problem for telescopes. Even as little as 5 microinches would be a noticeable degradation of performance for most telescopes, and as little as 1 microinch would be devastating to the Hubble Space Telescope. Yet the Palomar telescope has been in service for over 50 years without so much as 5 microinches of flow. Since you would need at least 10,000 microinches of flow to easily observe the effect, that means at least 100,000 years would be required at the same rate of flow (assuming that glass flowed just fast enough not to be detected on Palomar yet.)

So what gives with those old windows in buildings and cathedrals? The fundamental flaw in the observation is that glass was not made of uniform thickness when those building were glazed the way glass is today. Today float pans are used in which molten glass floats on a molten layer of tin. As long as sufficient care is taken, this produces glass of very uniform thickness and glass that is very flat.

However, before this method was developed, glass was shaped more or less by hand. This resulted in pieces that were noticeably thicker on one end than on the other. Usually installers tried to put the thicker glass at the bottom of the window frame where it was felt that it would be more stable.

For further discussion on the subject, try doing a search for Robert H. Brill, Research Scientist, The Corning Museum of Glass. You should find a paper he wrote in July of 2000. See also this site and this discussion.

One of the things that telescope owners do have to worry about is gravity sag. This is an elastic deformation that is proportional to the gravity load and disappears when the load is removed. Since telescopes generally have to be re-oriented to see in different directions, this becomes a problem to control. One French approach was to build the telescope so that the glass, deformed under its own weight, was correct in a horizontal orientation. The pointing was then accomplished with a larger turning flat mirror. Thus the main telescope mirrors were always in the “correct” shape. Of course, if gravity had caused the glass to “flow,” this approach would not have worked.

19. Clouds float in air because the droplets are tiny.

Actually, that plays a role. It should be obvious that large drops would fall immediately. We assume here that we are in the earth’s atmosphere and that gravity is doing it’s usual thing. OK, so tiny droplets is part of the explanation. Another part is that air molecules are constantly bombarding the tiny droplets so that it is difficult for them to fall. Hence they fall at a MUCH lower rate than the simple acceleration due to gravity. [Notice I said “fall at a rate” – this is imprecise, since things do not fall at a constant rate until they reach “terminal velocity.”] So, if nothing else were in effect, a cloud would descend pretty slowly due to the water droplet terminal velocity. However, as the sun shines on the cloud, quite a bit of sun light is absorbed as heat and is transferred to the air in the cloud. This tends to make the air in the cloud rise. This updraft tends to pull the water droplets along with it. Again, this would not be possible if the droplets were much larger, unless some pretty violent updrafts were present (such as in a hail storm.)

20. Relative humidity is ratio of the moisture in the air to how much the air can hold.

This again is very imprecise language and usually leads to misunderstandings. Generally, unless the pressure gets too high, or some sort of chemical reaction is taking place, how the components in a gas (such as the air) act individually is insensitive to the other components in the gas. For example, the air molecules do not actually hold the water – the water would pretty much do the same thing if the air were not there. So, at a given temperature, water vapor will come to equilibrium with liquid water (by that we mean the rate that water is evaporating is equal to the rate it is condensing) at some vapor pressure of water. This is the same regardless of the air. Relative humidity then represents a state of non-equilibrium. This means that something has swept out some of the water so that less water is in vapor than could be at the given temperature. Alternatively, the area may have just heated up, and the water has not had a chance to catch up. Or there might be very little water around to evaporate and reach equilibrium. This explanation is itself a bit of a simplification. In a state of non-equilibrium, the rate of evaporation can be controlled by the air molecules. They sort of get in the way, hitting the surface of the water and slowing the evaporation. If they were not there, the liquid water would boil until the vapor pressure of water above the water reached the equilibrium pressure for the temperature of the water. Of course, the evaporation process takes energy, and this cools the liquid water. I have had difficulties removing water from a vacuum system, because the water quickly freezes and then starts a very slow evaporation process. I have had to re-pressurize and re-evacuate a system over and over to try to remove water, and organic solvents. But, weathermen and scientists who know better, will often just use the short hand of “how much the air can hold” to describe the equilibrium partial pressure of water vapor.

21. Something weighs less in air than in a vacuum due to the buoyancy of the air.

This, of course, is shorthand. Does a rock weigh less in water? Is a ship weightless in water (it does not sink at all)? You probably at least agree that the ship weighs the same in water as on land. If you think about it, the rock weighs the same too. What might be confusing, is that the force to lift the rock in the water is now less than its weight. Strictly speaking, the force to lift the rock is the same. It’s just that the water is exerting some of the force on the rock, just like it is exerting all of the force to hold the ship up on the surface. In the same way, air pushes up on objects and makes them appear lighter. Would you say that a helium filled balloon or hot air balloon is weightless, or negative weight? Probably not. So in the same way, saying something “weighs less in the air” is incorrect and misleading.

22. Is the air weightless.

Of course not. If it were, it wouldn’t stay in place around the earth. In fact, all of the air above every square inch of horizontal surface weighs about 14.7 pounds.

23. What do raindrops look like?

Actually the exact shape depends on how big the drop is. Smaller drops are sort of egg shaped. Larger ones get to be sort of Kisses™ candy shaped with the point removed. There really is no point at the top of the drop as you so often see it drawn.

24. Infrared light is “heat waves”

First, infrared is not “light”, so it should be called infrared radiation. More importantly, heat is a property associated with matter, not radiation. Heat content of matter is a measure of the internal energy states that have been occupied. So heat does not equal radiation. But why would people say so? For warm objects on the earth (we mean something warmer than freezing and cooler than say a candle flame) the dominant form of radiation is infrared. This means that a warm object will be constantly giving up some of its heat in the for of infrared radiation. By the same token, if it is in equilibrium with its surroundings, it will be receiving about an equal amount of infrared radiation and absorbing it and converting it back to heat. This expression may have been used for “heat lamps” which radiate predominantly infrared and tend to keep objects hot place under them. It may have been used to describe radiant heating systems for homes. At any rate, it is another shorthand expression that can not be taken literally. If you are confused by the term "heat wave", do not feel bad. It is misused almost constantly so that its meaning is really unclear.

25. Are the colors of the rainbow “red orange yellow green blue indigo violet”?

Actually, there are many more colors than that. But it is the blue-indigo-violet that is typical drawn incorrectly. The "blue" in the rainbow that people usually is in real life cyan, and the indigo and violet are really dark blue. There really is no purple in a proper rainbow. Violet is really extreme blue. Purple only occurs where multiple rainbows overlap.

I have recently discovered that there is a minutephysics video on this subject here: colors of the rainbow.

26. The north magnetic pole is at a location shown on a map

Well, if you realize two things. The north magnetic pole is opposite to the “north pole” on a compass. Some textbooks like to call the compass the “north seeking pole” So labeling the “north” and “south” magnetic poles has become the choice of the author of a textbook. Ugh. Need for standards.
Secondly, the effective north magnetic pole is hundreds of miles below the surface of the earth. Therefore, one should bear this in mind when thinking of the earth’s magnetic field.

27. Laser light is perfectly “in phase” and parallel

These are aspects of the coherence of a laser beam. Strictly speaking, a beam can only be perfectly in phase and parallel if it occupies an infinite amount of space and time. Real laser beams only last a finite amount of time and are only “so wide.” Roughly, the angle at which the laser deviates from parallel is at least the wavelength divided by the diameter of the beam (measured in radians.) Real lasers have other physical limitations that prevent them from achieving even this degree of parallelism. But some lasers can come very close. A typical pocket laser pointer will have a divergence angle of about one milliradian – about one fifteenth of a degree. This means that it would form a spot on the moon about 150 miles across. This is obviously not perfectly parallel, but pretty close for everyday work. Another shorthand expression. Lasers are not perfectly in phase, of course. The coherence length is a measure of how far a laser beam can go before the phase of the beam deteriorates so much that it is practically out of phase with itself. That is imprecise language. What we really mean is that a laser beam going through two slits will form an interference pattern that is bright/dark where the modulation is close to 100%. After one coherence length, that modulation falls to something like 30%, and it gets worse to where eventually, you get no interference at all.

28. String theory, dark matter, dark energy, m-branes, multiverses, supersymmetry, brane worlds, M-theory, the anthropic principle

OK, these topics now cross into the realm of "unknown." I cannot definitively say whether string theory is wrong, but is really does not qualify as a theory in my thinking.

Despite the fact that Coulomb's Law was later found to be wrong (it does not describe the forces between charges that are moving, nor does it account for the speed of light), and despite the fact that Newton's Laws were discovered to be wrong in the situations governed by relativity, we have a rather strange reaction to finding out that there is something wrong with out formulation of gravity.

It was observed in the 1930's that the rotational rate of stars in the outer portion of spiral galaxies were rotating faster than Newton's Laws of gravity could predict, given that the matter in the galaxies was what you could see in terms of stars. Rather than keeping an open mind that we might have something over-simplified in Newtonian and Keplerian mechanics, we seemed to have leaped to the conclusion that there must be some magical form of matter out there that doesn't interact with regular matter, we can't find any, we can't see it in any experiment, but it is there, it has gravity (but nothing else) and what's more, it is most of the universe, not the regular matter. Sound ridiculous? It should. Even if it was suggested by Albert Einstein and Willem DeSitter in 1932.

The other so-called theories are just as shaky, with little basis for them other than they represent one of many possible explanations for the observed physics, and some powerful figures in science endorse them.

We seem to have forgotten out embarrassments over N-rays, polywater, water memory, the gelatin x-ray laser also here, cold fusion, and magnetic monopoles. (When I was in graduate school, there was a big excitement over magnetic monopoles, and new versions of Maxwell's Equations were developed to handle the case for divergence(B) ≠ 0. I also was following the x-ray laser development which was derisively called the "Jello laser." Unfortunately, it did not produce x-rays.) These new theories in cosmology are big business but lack any of the characteristics of known physics, despite being published and shown on television as established fact.

For a little tiny flavor of the the confusion, speculation and near nonsense, take a quick look at this video.

I downloaded some string theory conferences (check here - many of the past conferences can be downloaded), and I had to follow at least half a dozen papers before I started to get what they were talking about. String theory doesn't really work well in our universe, so they work in Anti-DeSitter space of 5 dimensions (which they call ADS5), (yes, the same Willem DeSitter) where the math works particularly well. Sometimes they work in up to 25 dimensions. Yes, they know that is is not our universe. It doesn't bother them. They just started to say, well, maybe we have more dimensions that we just don't know about yet. Then they started to believe this conjecture. Are dark matter and dark energy consistent with this thinking? Uh, well, not a good time to ask. Could dark matter simply be multiverses that are wrapped up in back hole bubbles that we can't see directly because light can't get out?

For further reading, I recommend you take a good long look at the following books.

29. 97% Caffeine Free

This is an old one, and forgive me because I was taken in by this as a teenager, but when they advertise 97% caffeine free, I expected that they meant they took out 97% of the caffeine that is normally in coffee. Actually they only took out half the caffeine. They thought "half the caffeine of regular coffee" made them sound like slackers, so the dreamed up the following logic. Regular coffee is 6% caffeine. That means that it is 94% caffeine free, right? So if we take out half the caffeine, we can call it "decaffeinated" and say it's 97% caffeine free, right?

Admittedly, this one doesn't take you in today, because the ploy has been copied so far and wide that you have to be living under a rock not to have come across it by now.

30. This teeth whitener or clothes whitener is safe because it only uses oxygen

31. Ring of Fire spark plugs

32. Power factor phase electronic gizmo saves on your electric bill by cheating the electric company

32. Permanent magnets soften your hard water

The claim is that the magnesium and calcium in hard water can be put into some kind of magnetized state that prevents it from precipitating out in your water heater, plumbing, washing machine, on your bathtub tile, your dinner dishes, etc. There are wonderful testimonials by responsible people who believe that their system is as clean as can be and that the lime/scale has even vanished by using these magnets.

Please be aware that I know physicists and engineers who own these systems (you don't have to be a layman to be fooled.) Every controlled test has shown that these magnets do nothing. No independent test has ever shown the slightest benefit from the magnets or that any of the claims are actually true. Now when they start to tell you that well, in the real world it works, but it doesn't work in the independent tests, you should stop listening and walk away. I am sorry, there is no magic. Now if you want to believe and give this guy your money for some useless junk, that is your business. You have been forewarned.

33. Soda bottle stopper with pump keeps soda from going flat

It seems like this would work, right? You notice that the soda seems to stay fizzy with that pressure that is released when you open the bottle. So it makes sense that if you add the pressure, it will stay fizzy. Unfortunately, no. What matters is the partial pressure of carbon dioxide gas about the soda. Since your little hand pump is going to only add air, which has almost no carbon dioxide in it, you are not going to do anything.

There is a small exception to this. Those thin plastic soda bottles that tend to expand when the pressure builds up inside can benefit (only slightly) from the pressure inside by reducing by a very slight amount the volume of carbon dioxide it takes to reach an equilibrium pressure with the dissolved carbon dioxide in the soda. It is very probable, however, that you'll never notice the difference.

34. Stick on cellphone antenna

35. High gain antenna improves wifi coverage

36. Powerful binocular optics for only $10 (plus postage and handling)

37. Lasik eye surgery results in near perfect vision

38. Low Power Laser Hair Regrowth

39. Little Battery Operated mysterious hair remover

40. "Genuine" Diamels, DiamondAura, etc.

41. Magnetic blue laundry magnets

You may have seen an advertisement on Facebook, or in some email, or featured on some shopping site. This is closely related to the permanent magnet water softener. Based on some unexplained principle, these mysterious magnets get your clothes clean without detergent. The sales pitch includes a lot of hype about how detergents are made from nasty chemicals that are to be avoided. You are instructed to wash your clothes with no detergent, possibly supplementing the magnets with some enzyme soil remover or vinegar.

Unfortunately, only selected portions of the independent test labs results are given. In particular, the comparison of washing in just plain water (with no magic blue magnets) to using the laundry magnets is not shared. Several people have made the comparison and posted results. The bottom line: no difference. And no, it is not just as good as with detergent.

Are we using too much detergent? Certainly. Detergent manufacturers typically specify about 30% more product than is actually needed. Are there alternatives? Yes, depending on what level of dirt you need to clean from your laundry, you can use white distilled vinegar, baking soda, borax, and/or castile soap, which usually do a credible job, but stubborn dirt will require good ole detergent and enzymes. Don't use vinegar and baking soda at the same time. This is the classic example of reacting an acid with a base.

By the way, several people in my family are sensitive to the chemicals in laundry detergent, so we use detergent free and clear of all coloring and scents. We also use (when needed) the enzymes that come in a deodorant-like push up container so that the product is semi-solid and is rubbed into the stain.

42. Magnets in ankle bands, knee braces, back braces or wrist bands

43. Copper infused clothing, copper wrist bands

44. Auxiliary lenses, lens converters

The reason this item is included is that in most cases, you would be better off without using these lenses. In other words, you could accomplish at least as good a job using other techniques in your smartphone such as panorama mode or digital zoom.

Laser and photon myths

46. Photons reflect from surfaces by being absorbed by atoms and then re-emitted in the opposite direction.

47. Photons go slower through transparent media because they are being absorbed and re-emitted by atoms.

What slows down the light wave? Not absorption and re-emission of photons. First of all, the photons are not absorbed by any atoms or molecules or electrons and re-emitted. The light frequency in a transparent material may be far from an absorption band. The electronic transitions are quantized, and they don’t absorb just any arbitrary wavelength of light: they absorb at the wavelengths that contain just the right energy. Let’s think about what would happen if atoms in the glass did absorb and re-emit photons.

The photons would not just be re-transmitted in exactly the same direction as before.
They would not be exactly in phase with the original photon.
Each photon absorbed and re-emitted would have a random amount of time delay before a photon was emitted.
Most of the atoms in an excited state would collide with another atom and transfer the energy into the medium. The quantum of energy usually transferred into the medium for solids is a phonon, and it eventually ends up as heat. It would not hang around long enough to re-emit the energy as a photon.
A laser beam going through glass would get diffused and lose coherence because of all the random changes in phase, direction and time. You would send a laser pulse in and light would keep coming out for a while after the laser pulse stopped.

It just doesn’t happen that way in transparent material. That is not a viable explanation of what is going on.

If they are teaching this stuff in school, I apologize. They are probably giving you an explanation that is easy to make it on the assumption that you will buy this answer and move on. Unfortunately, it is junk science.

It is all about driven oscillators. What actually happens is that the e-m wave enters a material (which as I said does not absorb the light because it has no oscillators —electronic, atomic, or molecular— that absorb that frequency) and the electromagnetic wave drives the atomic oscillators. By atomic oscillators I mean electrons bound to atoms. Because the electrons are very low mass, they can easily move at high frequencies from one side of the atom to the other. (They do this without absorbing a photon.) However, since the electron bound to the atom does have a characteristic oscillation frequency determined by the forces holding the electron to the atom and the electron’s mass, the electron is being driven well below its natural oscillation frequency. This means that it responds at a slightly different phase from the original light wave coming in. The resulting light wave is the combination of the incoming wave and the wave created by the electrons driven from side to side around their atoms. The combined wave has a small phase lag.

One atom in the light wave is going to have a small effect on the overall wave. But as we add more and more atoms, the effect on the resulting light wave in the material becomes more pronounced. As the light wave propagates through the material, the phase is getting more delayed the more it travels. The more atoms per cubic centimeter, the more the entire phase velocity is slowed down. The refractive index is a measure of this phase velocity. Different materials at the same density are going to have different amounts of slowing of the phase velocity of light because the forces which hold the electrons to the atoms are different, but for a given type of atom, the more per cubic inch will have a greater effect on the light wave going through.

Resonant effects. If the light frequency gets close to the natural frequency of the atom-electron combination, strange things can happen. The light can be absorbed and eventually become heat. That isn’t so strange, but near the natural frequency, the phase of the driven oscillator can contribute to the original wave to advance the phase of the light. This would make the phase increase faster than the original light wave, resulting in a phase velocity greater than c, and an index less than 1. This may seem strange to you, but it certainly happens in some materials.

The group velocity will never be greater than c in any material, in fact it will always be lower than c in a material. On the other hand, near the natural frequency of an oscillator in the material the phase velocity could be greater than c. Hence near resonance, the phase velocity and the group velocity can be significantly different.

Non-resonant light velocity. Far from the natural oscillator frequency, such as is the case with light propagating in glass, the phase velocity and group velocity will be virtually identical and always less than c, and the greater the density, the lower the phase and group velocities, or put another way, the higher the refractive index.

You can measure the refraction angle, and the Brewster angle of the material. These tell you the phase velocity of light in the material. You can also propagate pulses of light through the material and measure the time delay compared to a pulse traveling through a vacuum. This will tell you the group velocity in the material.

Recommended reading:

The Origin of the Refractive Index

(from the Feynman Lectures, Volume I Chapter 31)

Edit: in the world of quantum electrodynamics, light is made of particles, not waves. It is just that these photons obey Maxwell’s Equations with the understanding that the intensities predicted are the probabilities of finding a photon.

This means that in QED theory, the photons ARE absorbed and re-emitted by the electrons in the substance. However, each photon is absorbed and not absorbed simultaneously by all the electrons in the substance. In other words, each photon takes all possible paths to get into and propagate through the substance. Note that you cannot say a photon goes along and is absorbed by an atom and reemitted and goes to the next atom. That is a gross misinterpretation of QED. These are quantum particles. They do not behave classically. If you can understand how a photon is simultaneously absorbed and not absorbed, you are on your way to understanding light and quantum mechanics.

48. Lasers are absolutely monochromatic.

First things first. Laser light is not perfectly monochromatic. All the photons are not the same wavelength and they are not all in phase.

Laser light can be quasi-monochromatic. In this case, it is a very narrow bandwidth, typically from a few kilohertz to a few megahertz. Lasers can also operate on several transition lines simultaneously. There are white light lasers.[1]

Let’s assume that we are talking about a laser that operates on a single quantum transition line. So how do lasers achieve such narrow-band light?

First, the transition line. As you probably know, a particular kind of atom (or molecule in the infrared) may have a transition from one state to another state with a very specific energy difference. This energy difference is determined by quantum rules and is a very exact amount of energy difference. If all the atoms are exactly alike, then the photons will all be exactly the same frequency, in the frame of reference of the atom.

Classic transition line discharge lamp is a sodium vapor lamp operating on the yellow line.

However, the atoms are all moving at different velocities in different directions, so they, in reality, from the frame of reference of the laser device, have Doppler shifted frequencies or wavelengths. This is true even in a solid state laser, because the atoms are vibrating within the crystalline lattice or within the amorphous glass.

For gas lasers, there is also the issue that atoms tend to collide during the process of emitting a photon. This causes the phase of the emission to be randomly changed during the emission. If you have a model in your head of photons being whole particles that cannot be divided, then you are not going to do well in the field of lasers. You can think of light as waves, or you can go the QED (quantum electrodynamics) route and think of them as taking every possible path between events according to probability rules. Either way, photons are very bizarre particles.

At any rate, the frequency of the transition is generally broadened by Voigt profile, which is a fancy name for a combination of the pressure broadening (due to collisions) and the Doppler broadening (due to atomic velocities).[2]

Voigt frequency profile

So the atomic transition line turns out to be, say, about half a gigahertz to a couple of gigahertz wide. This is the same width you get out of a gas discharge lamp, like a neon sign.

How does a laser make a narrower line?

The laser includes a resonator cavity. In its simplest form, it is two mirrors. The two mirrors can act like the pipe in a pipe organ. It results ins a standing wave which works like this:

Standing wave. If there is not an integral number of wave for the whole path of one round trip, then the wave will partially cancel itself. Wavelengths that are an integral number will reinforce themselves. They are called “modes.”

Since there is a standing wave in the cavity, there must be an exact whole number of waves between the mirrors if the mirror reflect 100%.

In general, however, there are a large number of waves between the two mirrors. Say the number is n. But n+1 waves and n+2 and n+3 and so on are also allowed. Each one of these is a different axial or longitudinal mode. (Sometimes they are called cavity modes.) They are all equally valid and slightly different frequencies. As long as the frequency is within the Voigt profile, the laser will work at that frequency.

Typical cavity lengths and atomic lines will allow a few to a few dozen of these axial modes, so the laser output would be several discrete frequencies. That is if the mirrors were perfect reflectors and perfectly flat. Since there are no perfect reflectors (despite hyped-up claims in the literature), these discrete lines actually have a finite width, usually about a megahertz or so. Overall, the laser output is going to still be of the order of gigahertz wide.

How do we narrow it further? We can design the resonator cavity length to be short enough that only one longitudinal mode operates. Unfortunately, that is usually very inconvenient. But we can put a sub-resonant cavity inside the laser cavity with a very short spacing between two partial mirrors. This can be tuned to only one longitudinal mode. This resonator is called an etalon.

Etalon

With this filter we now can get down to a megahertz or better. We get a lot less power from the laser, because now we are only using the atoms that happen to be going at the right velocity to contribute to our laser beam.

So how do we make the laser narrower? We can be very careful in designing the etalon. As high a finesse[3] as possible, with control of vibrations and thermal changes in the etalon. We can also use multiple etalons working together to get very narrow lines. We can get down to about a kilohertz or a bit less.

Spectra of etalons with different finesses. The yellow line is the highest finesse and the blue is the lowest finesse. Since the separation of the etalon modes is greater than the laser transition width, only one etalon mode will be active.

Footnotes

[1] Bill Otto's answer to Do white lasers exist?

[2] Spectral line shape - Wikipedia

[3] Fabry–Pérot interferometer - Wikipedia

49. The photons in a laser are in the same state.

Most of what you think you know about lasers is false.

Lasers can (but aren’t necessarily) be very collimated (nearly diffraction limited) and very narrow band (quasi-monochromatic.)

Such light can be easily focused to a very tiny spot of high intensity or can travel hundreds of feet before diverging.

If a laser beam is expanded in a high quality telescope, it can be focused hundreds of kilometers away on a spot not much larger than a basketball.

Lasers are used to measure the distance to the moon, however, the spot on the moon may be a few kilometers wide. To do better we could use adaptive optics and a 3.5 meter telescope and get a spot only about 200 meters in diameter at the moon’s surface. But that’s about the limit for now. Get a 100 meter diameter telescope and we can get that spot down to less than 10 meters, about the size of a one car garage.

The thing that most people recognize is that most laser beams are collimated much better than ordinary light sources. You could collimate regular light like that, but the efficiency would be so tiny that it would be ridiculous. A laser pointer spreads at an angle of 3 to 5 arc minutes. A very high quality flashlight will spread at 5 to 10 degrees. The main reason is the the flashlight needs to be small and not waste too much light.

High quality flashlight beam.

Laser pointer beam.

A laser resonator on the other hand can filter out light of the wrong direction or wavelength, and because of amplification, only the light getting through the filters is amplified. In this way, we can amplify the narrow band or the collimation of the light. It is so strikingly more collimated and narrow band than ordinary light sources that people have said it is “monochromatic” “perfectly in phase” or “goes on forever without diverging.” None of these is true.

People also say it is “coherent” which is true, but then they describe all the photons as marching “in step.” This is not true. For more information on coherence and phase of lasers, see Bill Otto's answer to Is a laser beam still in phase if you bounce it off a reflective, but non-mirror surface?

Is a laser beam still in phase if you bounce it off a reflective, but non-mirror surface?

This is another question with a lot of assumptions. A laser beam is not “in phase” to begin with. At least probably not in the way you think it is.

I have explained this over and over.[1] A laser beam is not monochromatic. It is only quasi-monochromatic. That means all different phases are present at the same time.

OK, so how do we get an interference pattern when we build an interferometer? Well, if we split a laser beam into two parts with a very flat beamsplitter so that the phase going through has a very constant relationship to the phase reflected, then each part of the wave interferes with its exact counterpart. If the pathlengths of the two paths, including the beamsplitter thickness are equal, even broadband white light will interfere. It is not easy to get the paths equal to a tenth of a micron, but it is possible. I did this experiment in 1984 because I had to prove it to some managers who wouldn’t believe anything unless it was demonstrated in our laboratory.

This is a simple “white light” (broadband) interferometer. I did not have a beamsplitter cube, so I needed a compensator plate, but the principle is the same.

I used “white light” from a tungsten lamp. The coherence length of such light is only about 0.2 microns. It isn’t what most people would call coherent. But it could be made to interfere with itself (the other half from a beamsplitter) as long as the beam was well collimated and the pathlengths were nearly equal. I also could not get very much beam shear, because one part to the beam would not interfere well with another part of the beam. So this beam lacked spatial coherence and temporal coherence.

Fringes from a tungsten light source. These are much prettier than my fringes because these were made with carefully matched beamsplitters. I had to make do with two beamsplitters I could find in my lab.

Now what if I take an extremely bright tungsten lamp and focus it through a very tiny pinhole? And then re-collimate the light. It now has spatial coherence. That means the left side of the beam can interfere with the right side. Again, the beam is not “in phase.” It is just that all of the complicated mixture of phases on the left side of the beam is the same as on the right side.

I actually did this experiment in 1984 as managers gathered around stunned by what was being shown. I next got a very narrowband interference filter that I had ordered from someone I knew at Optical Coatings Laboratory, Inc. and put it in the tungsten light beam.

My narrowband filter actually made better fringes than shown here. Because of non-disclosure agreements, I was not allowed to keep any of my pictures.

Now the beam would interfere with itself, even if the pathlength difference was a few centimeters. I now had spatial and temporal coherence. This was better than our chemical lasers could do. That was because they operated on many vibrational and rotational transitions and had almost no temporal coherence.

Next I spatially filtered the light from an argon-ion laser. I put an etalon in the laser to “spoil” all but one of the electronic transitions and “modes” of the laser. Now I had a beam with a (temporal) coherence length of more than a meter. The interference fringes were very high contrast. Did that mean that the laser beam was all in phase? No! It meant that the complicated mixture of phases was correlated laterally across the beam and longitudinally along the beam for over a meter.

Argon ion fringes at the 514 nm line from an argon ion laser. Lower contrast concentric ring fringes are from unintentional reflections from the lenses used to collimate the beam. Notice that laser beams tend to be brighter in the center of the beam than on the edges.

So I am going to assume that this is what you mean when you say “the beam is in phase.” You mean that the complicated mixture of phases is correlated across the beam and along the path of the beam.

If you reflect such a laser beam off a flat, optically polished dielectric such as a glass beamsplitter, the phase correlation will be preserved, and the beam will still have the same spatial and temporal coherence as before.

If the material is a rough surface, or a diffusing dielectric material, or a Rayleigh or Mie scattering volume such as air or fog, then the light will scatter in all directions, mixing left to right, up to down, with different path lengths. The phase correlation will be destroyed by this process, and the spatial and temporal coherence of the scattered beam will be greatly reduced. Typically, most people will notice that the reflected beam exhibits a lot of speckle. I have written about this, also.[2]

Laser speckle caused by random interference of a coherent beam reflected from a rough surface.

Footnotes

[1] Bill Otto's answer to Is a laser beam composed of photons found in the same quantum state?

[2] Bill Otto's answer to Why does laser light look grainy?

50. The photons in a laser are all the same wavelength.h.

51. The photons in a laser are all in phase.

52. The photons in a laser are all going in the same direction.

53. The photons in a laser are entangled.

54. Light does not change frequency when going into a transparent medium.

55. Photons can propel a light sail and give energy to it without losing any energy of their own.

56. Making a a gigawatt laser is no big deal.

57. Blue light from entertainment device screens is dangerous

Blue light on smartphones does not cause macular degeneration. They just aren’t bright enough. By a wide margin. You have fallen for a scam. I am going to tell you how the scam works. I know a lot of you won’t believe me. No problem. I don’t get any money either way. But if you are curious about how it works, read on.

Computer screens are dim compared to normal outdoor illumination levels. There simply is not enough light from a computer screen to cause harm.

Cell phone screens just aren’t that bright, either. You get more blue light from looking at almost anything outdoors than a computer or phone screen.

Bill Otto's Blue Light Toxicity Discussion

So many people are concerned about protecting their eyes from blue light
I have researched this as a special topic.

This topic required research, writing some MatLab code, and triple checking my work to make sure I was on the right track. It requires a little bit of science to understand the answer. For this reason, experience tells me few people will bother to read it, and even fewer will know whether to upvote it. That's why I give the executive summary.

Executive Summary

Recently tests showed that eye damage could be caused at below the "safe" thermal damage thresholds. The damage was being caused by very high intensity yellow-green light for long exposures. Later research showed that very intense blue light at certain wavelengths caused about twice as much damage as green or red.

The damage levels are about 200 Joules per sq. centimeter. For comparison, that is roughly the energy level you get if you take someone to a bright white gravel pit on the planet Mercury and force them to look at the sun-illuminated gravel for 500 hours straight. That would be the worst case of snow blindness you could imagine. For testing, they don't like to wait 500 hours, so they up the brightness another 30 times, and expose for about 16 hours.

This is the level at which they found cone cells dying from blue light toxicity. It has nothing to do with everyday light levels.

This is like saying automobile seats are dangerous because in airline crashes at 300 miles per hour, seats that were made of less than 1 inch thick titanium broke about half the time. Therefore, you should replace your automobile seats with 1 inch thick titanium frame seats. Unless your car is in a 300 mph wreck, that seat is completely over-designed and mostly useless.

Blue light is necessary to your well-being. It is associated with not only seeing well, but being emotionally well adjusted. It is necessary for synchronizing your Circadian rhythm with night and day. (Even the blue-blocking lens people admit that. See footnote 7.) Claiming that blue light is dangerous and then selling you blue light filters is a total scam.

The beginning of "Light Toxicity"

This figure is on a logarithmic scale. On this type of scale you can show the size of an atom and the size of a planet on the same scale. Notice that using a computer exposes you to about a million times less light than the MPE (maximum permissible exposure level.)

The maximum permissible levels exceeded a watt per square centimeter for small spot.

They exposed a retina to very high levels of yellow-green light (568 nm) - near the maximum permissible level. They observed the following damage:

from Light-induced retinal changes observed with high-resolution autofluorescence imaging of the retinal pigment epithelium. Morgan J.I., Hunter J.J., Masella B., Wolfe R., Gray D.C., Merigan W.H., Delori F.C., & Williams D.R., Invest. Ophthalmol. Vis. Sci. 2008 Aug; 49(8):3715-29.

Note the energy levels. These are equivalent to exposing the piece of retina to direct sun viewing for 8 - 30 minutes.

This was reported in The susceptibility of the retina to photochemical damage from visible light

This is no real surprise: at extremely high levels, light just overwhelms the cone cells and can actually kill them. These researchers expected that cells would all die at about the same level of sunlight. In other words, they expected that if the green light in the sun was three times stronger than the blue light, that the cells would die at about the same rate if exposed to 3 times less blue light. What they found was different. The cells actually died at about twice the rate for extremely intense light in the range 430nm-460nm. But the intensity was 30 times brighter than looking at snow-white gravel on the surface of the planet Mercury. That's about 300 times brighter that looking at snow on the planet Earth. So what did they prove? If you look at something 300 times brighter than snow on a clear sunny day for 16 hours straight, there is danger from blue light. Well, duh, there is danger from other colors of light at that intensity also. This is the level you get if you try to look at a solar eclipse through an unsafe make-shift filter of cross-polarized filters.

Here is the chart that they usually use to justify the purchase of a blue light filter on your glasses. See how blue light kills more cells than other colors. They neglect to mention at what intensity blue light kills cells.

So what do the blue light filters do? Basically nothing. They are designed to be deceptive. They reflect about 20% of blue light - just enough so you can see a blue reflection and think that they are actually doing something.

This is the actual advertised transmission curve. Only 20% of the blue light in the so-called dangerous blue wavelength zone is blocked. That is not enough to do anything. They claim that at the super bright test intensities, it cuts cell deaths by 25%. But what does that have to do with using a computer screen a million times dimmer?

This is the clever marketing gimmick. You can see the blue light being reflected. Are you convinced yet? Want to buy these coatings just to be safe? Don't forget to go get your 1 inch thick titanium car seat frames "just to be safe."

Here is another type of image you might see in an advertisement. This is 100% false. If blue light were blocked like this, your lenses would be very yellow indeed. This is just a photoshopped picture to try to motivate you to buy.

What do these coatings do to computer monitor light? Here, let's run the computer light through their filter.

Here you can see that the light is reduced from about 0.016 mW/cm^2/100nm down to about 0.014 in the blue peak. You could get the same reduction just by turning down the brightness on your monitor a tiny smidgen.

I based the curves on a 250 candelas per square meter monitor. To be dangerous, you would need about a 250,000 candelas per square meter computer monitor. Just be sure you don't buy one that bright (hint: you can't buy one that bright because they don't make them.)

These blue mirror finish sunglasses actually do reflect enough blue light away to make the world appear yellowish. These are not sold as blue blocking coatings, but they actually accomplish just that. At first I thought the sunglasses were tinted yellow, but looking through them at grazing incidence allowed blue light to pass through.

By the way, you can install an app on your computer or your smartphone to reduce blue light at night. See footnotes 3, 4 and 5.

Note: I include in the footnotes the articles in which they try to scare you into getting their blue blocking coatings. Don't fall for it.

Why do they try to sell you practically useless coatings?

Mostly because they would go out of business if they did not find ways to up-sell. It is expensive to rent and operate an optician shop and provide competent consultants to help you pick out frames and advise you on the types of lenses to get. Sadly, this advice is far more expensive than manufacturing the glasses themselves.

To offset these costs, they have no choice but to look for every opportunity to up-sell. Designer frames, name-brand lenses, and expensive coatings are simply ways of making ends meet. For an inside look at this world, check out this article. Everything to Know About Blue Light and Crizal Prevencia

Footnotes

[1] 3 Blue Blockers Put to the Test

[2] Computer glasses that claim to protect your eyes from screens are selling like crazy, but they probably aren't doing you much good

[3] Twilight - Android Apps on Google Play

[4] 5 Best Android Apps that Reduce Eye Strain for Night Reading

[5] How Can I Make My Computer and Phone More Friendly to Use at Night?

[6] The susceptibility of the retina to photochemical damage from visible light

[7] Light can be both harmful and beneficial to your vision and health

[8] Blue light hazard - Essilor

[9] Characterization of light toxicity on retinal ganglion cells in vivo and in vitro

[10] Combined Use of In Vitro Phototoxic Assessments and Cassette Dosing Pharmacokinetic Study for Phototoxicity Characterization of Fluoroquinolones

[11] Characterization of the blue light toxicity spectrum on A2E-loaded RPE cells in sunlight normalized conditions

[12] Blue light excited retinal intercepts cellular signaling

[13] Everything to Know About Blue Light and Crizal Prevencia

This jibberish is the opinion of Bill Otto and does not necessarily represent the views of any scientific organization.
Copyright © 2005,2017 [Bill Otto]. All rights reserved. Certain graphics have been lifted from other web sites. If you are the copyright holder thereto and want credit or want me to delete the image, please email me. I have completely re-written the html code to be more mobile friendly, but I am not a web designer. The website is best viewed with a laptop.
Revised: January 28, 2019 .

BAD SCIENCE