Dalo

Agreed. I will just mention in passing, without in anyway inviting a new discussion, that there is a continuum with, at one end, observations for which we seek an explanation, as is the case for the color of the sky, and, at the other end, un-observed or even unobservable phenomena which the theory predicts, as most facts are in, for instance, Quantum theory. Regarding the color of the sky the following quote of Raleigh might be of interest: "It is now, I believe, generally admitted that the light which we receive from the clear sky is due in one way or another to small suspended particles which divert the light from its regular course... Whenever the particles of the foreign matter are sufficiently fine, the light emitted laterally is blue in color... the standard of linear dimension, with reference to which the particles are called small, is the wave-length of light." John William Strutt, On the light from the sky, its polarization and its color. 1871. (freely available on the web) I find these lines particularly interesting because they present a means of testing the theory beyond Earth atmosphere, on Mars for instance. Would particles of the same size as found in our sky yield the same color in Martian sky? I am genuinely curious as to the answer.

You say prediction, I say description.

@HallsofIvy Thank you for the clarifications.

This question got lost in the discussion that followed. I think they are both related and it might help making things clearer, or at least more concrete. Just to be clear, I wouldn't know how to answer it, so it is a genuine question.

It is related to the dormitive property of opium, an expression Moliere used to indicate that doctors in his time used very complicated jargon to express truisms. Maybe it does. My impression is that the calculations show that in one case the sky is blue, and red in the other. They represent the mathematical expression of familiar observations. They represent empirical facts and not theoretical conclusions. And the most striking property of an empirical fact is that it just is the way it is. A theoretical fact assumes that something could not be otherwise based on other empirical, logical and theoretical facts.

Why are you being so hostile?

I am sure it does. Analyzing empirical facts is an important aspect of physics. The second part is based on all kind of calculations while the first part is a simple observation. But then, the question is whether the second part is an explanation of why the sky is blue, or an explicitation of how it comes that the sky is blue: it is blue because short waves are more efficiently scattered than longer waves. This is a dormitive property. It does not however make the calculations any less valid. It does not explain how the sky could not possibly be any other color than blue. That would be an explanation.

The dormitive property of opium (Moliere, "Le malade imaginaire) will remain for ever the most acerbic critic of "savants' disguising their ignorance behind incomprehensible jargon. I would certainly not consider John William Strutt, better known as sir Raleigh, among one of the pompous 17th century characters, since I am sure that his mathematical calculations amply justify his reputation. Still, I cannot but wonder at the way his work is often presented. Take for instance this introduction from a respected site: "The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum." I would say that it is a pregnant example of dormitive properties. As it is here explained, the sky is blue because... the sky is blue. or more explicitly, because the blue light is scattered more efficiently than other colors. Am I glad this puzzle has been solved!

I wonder what the spectrum would look like on Mars through a prism. Anybody knows?

The concept of polarization is an established phenomenon associated with the theory of light as a wave. It is all about vibrations and their direction. It is also quite incompatible with Huygens' Principle which states that every element of a wave contributes to pushing the wave indefinitely until it meets an obstacle. Huygens was quite adamant about the fact that side processes did not diminish the energy necessary for the wave to keep going forward. It was, as far as he was concerned, the only reasonable assumption that did justice to the idea that light could travel indefinitely through space. Later on, those side processes, or "wavelets", acquired more pronounced features under Maxwell and those after him. They become identified as an electric and a magnetic field perpendicular to the direction of the light beam, and also at right angle from each other. It was those fields that made it possible for a wave of light to go on indefinitely. Those fields were themselves conceived as waves and we immediately wonder about their reach. Looking at a beam of light moving horizontally in front of the observer you can but wonder how far those fields reach. After all, we can see those fields from very far away from aside, so they must somehow be emanating some kind of particles that make them visible to the eye even from very large distances. A very straightforward explanation is that the beam is made visible through the scattering effect of dust particles in all directions. That would mean that the forward direction of the beam is at the same time accompanied by side waves that also reflect light in all directions sideways. Something Huygens found undesirable. It is also very difficult to understand how those fields could at the same time be moving forward and reach to infinity, or at least to large distances, sideways. Still, this is the view that is accepted by all scientists, and I will leave it to others to show me the errors of my ways. After all, I am not a physicist. This is also the view used by Sir Bragg in one of his very instructive black and white short films. where he explains polarization and the fact that a beam of light becomes sometimes invisible horizontally, and other times vertically, depending on how we rotate a Polaroid filter in front of the light source. I will only mention in passing my lack of understanding how vibrations can make objects visible or invisible, which is what Bragg is implying. When light is vibrating vertically we do not see the beam when we are facing it, but only its reflection on the mirror above, and vice versa when it is vibrating vertically. I wonder if it should be possible to make an object invisible just by the right choice of vibration. Also, I cannot remember any theory of vision that uses vibrations as a means of activating optical cells. Anyway, yesterday was quite misty, at least in the sky where no individual clouds could be distinguished. It was all wet and gray, and the sun could not be seen anywhere. That made me think of the way the light beam disappears in Bragg's experiment. Obviously the light is still present, since it was still day. But no sun rays or beams could be seen anywhere, the scattering of light being as perfect as can be. And I wonder if something like that is not responsible for the appearing and disappearing of the beam in Sir Bragg's experiment. After all, the light is still there overall. Even in places where it appears not to be.

I am not a physicist, but I find this very surprising. Even "invisible light" like X, gamma IR or UV rays are only known to us because we somehow succeed in making their effects visible.

Maxwell's Treatise on electricity never mentions light as anything else but an electrical effect. I think the assumption that light is not an em wave would not change anything to his equations. But please, remember, it is pure speculation. It is therefore more science fiction than science. I just wonder whether the assumption that light itself is an em wave is really necessary, and what it would mean for Physics if we assumed it is not. I think that it would make at least a very interesting intellectual exercise. Certainly not worth a Nobel Prize, but even if proven wrong, it would at least show which position this assumption takes in Physics and what the consequences are of denying its validity. I purposefully did not place this thread in a scientific sub-forum to avoid the accusation of propagating pseudo-science.

You may be right of course. I am not claiming any specific way through which light is created. The only general idea I would like to discuss is the possibility that light is a local phenomenon, an effect of em waves, but not an em wave itself. That does not mean I have any inkling as to how different substances react to em waves. Even assuming I am right, that would still be empirical issues that only scientific experiments and not pure speculation can address.

I found this old clip of the renowned scientist Bragg (junior) which sums up the dilemma quite nicely. It is interesting to note that the change of color can be seen on the screen, but not in the water. (around the 7th minute) I will leave the explanation to people more knowledgeable than me. Certainly not. I am just wondering whether light in general could not be a local phenomenon created by the collision of em waves and matter. Which would mean that light itself is not an em wave. This is purely speculative and I have no way of proving it I am afraid.

That is not much of an argument. Take all matter away and the light goes away too. I do not think Snell's Law would lose its validity if we assumed that the spectrum came from the prism.

I would have no problem with that. It is certainly not a matter of life and death. But could someone answer my question? Is is empirically provable that the spectrum does not come from the prism? Or is it a (necessary) theoretical assumption?

I will accept the scientific explanations given on the subject. What I am interested in is whether the assumption, if it is one, that colors come from the em waves themselves, and not from what they encounter, is a necessary assumption. I know it was Newton who was convinced that the spectrum could not come from the prism itself, and I wonder if this conviction is empirically provable.

Yes, that is the explanation usually given. I wonder: is it scientifically, empirically, proven that colors do not come from the matter particles themselves? Why assume that something is being stopped or let through, and not assume that different matter reacts differently to em waves?

and yellow brown on Mars?

You are referring to the process of image formation through a lens, a process in which rays, however they fall on the lens, cross each other at a focal point. This is a very understandable and well understood process, the quality of our optical devices from cameras to space telescopes being and undeniable proof of our technical know-how. You conveniently forget what the central issue is: if the sun rays reach the earth in a parallel way, only a very negligible fraction will reach our eyes, wherever we stand. And still, we see the sun in its totality. That is the puzzle I would like to see solved.

@Janus First, thank you for the drawings, they are very enlightening and impressive. They also show why reflection concerns the projection of one space point onto another single image point. They make in fact very nicely the abstract laws of reflection visible . Concerning the point where rays, or rather waves, become parallel with the distance. I think it is a fundamental assumption without which reflection would be unexplainable. Parallel light rays do create their own problems I am afraid. Imagine the Sun in all its majesty and... size. Its rays would start all entangled and criss-crossing each other, to slowly disentangle and become parallel. That is of course only possible if we enter the quantic field of wave-particles and forget about the abstraction of "ray" or even "wave". Only then can such an idea become meaningful. Back to the size of the sun. It is infinitely larger than our circle, and parallel rays/waves/particles can only mean that but a negligible fraction of those light elements, however we may want to qualify them, will reach the circle. That is also not a problem for our circle. As long as there are enough of them to create shadows we are covered, so to speak. It becomes more problematic when we want to explain the fact that we can, or at least think we can, see the sun in its totality (with the proper protections). Its sheer size becomes then unsurmountable. How come we can see the sun if its rays are reaching earth in a parallel fashion? So, I am afraid that we are back to square one: we have no problem explaining reflection as long as it means a one on one correspondence between object and image. It is when we abandon this principle that we encounter problems.

Thank you. I will study this very carefully and welcome any other information.

I admit that I still do not understand, so please bear with me. If rays were propagated in all directions, shouldn't the angles differ much more one from the other? If instead of statues we used thin sticks, wouldn't that make a difference? Also, a remark I placed earlier. When you look at the reflection of the sun on the water, you get blinded, but stepping aside solves the problem. If rays were propagated in all directions, shouldn't you get blinded wherever you stood. Here is a little "experiment" I made: shine an electric torch on the bathroom mirror. You will see the wide beam illuminating the mirror and its surroundings. You will also see the lamp itself of the torch, as a bright spot. If you move the torch around, the beam will describe a wider circle than the central bright spot of the lamp that will seem almost not to move. The rays of the beam will all move together in the same direction. Maybe the same thing happens with the sun?

Go ahead and say the same thing again and again. Imagine a unit circle (in meters). Put statues all around it, placing them on the perimeter. Make sure the circle is illuminated by the sun at some other time but real noon (the zenith). Now measure the angle of each shadow. I will admit that I have never actually performed the experiment, so I am really curious as to what the result would be. Will each shadow have its own angle (relative to some reference coordinates), or will all statues have shadows at the same angle? What could we conclude regarding the way sun rays are propagated through space?

You are still missing the point I am afraid. Forget about reflection since it seems to confuse the issue. Let us assume that we agree on it as an empirical fact and how it can be explained geometrically. The question is: knowing all that concerning reflection, how can we think that light rays are propagated in all directions? The last suggestion is: start from the subject, or his eye, instead of from the light source. I find it a very interesting suggestion and I would like to hear more of it.