Dalo

That is theoretically very relevant, and practically insignificant. Light that has been absorbed is out of the game. All we have left is the light that has passed through the gaps. Then "horizontal" is quite a flexible concept. When you are standing in front the water tank and the beam is visible in the water (and not on the mirror), you can still see the beam whether you bend your knees completely or whether you stand on your toes. That is exactly how I understood it: scattering is polarization dependent. If light is polarized vertically, it will be scattered vertically, and vice versa. My conclusion that the air molecules have no influence on the direction of scattering therefore stands.

I have reread the information on Land's polarizers, and also his patent . This is what I have understood once all kind of technical and chemical details are left out: 1) Very tiny crystals are arranged in linear arrays and separated by very small gaps (smaller than their width). 2) These crystal arrays absorb electric current (and light). 3) The gaps between the arrays let light pass. 4) The em waves which pass through the filter have (almost) no "wiggle room" in the direction perpendicular to the array. 5) The light that passes through can only be scattered in the direction parallel to the arrays. (Almost) no light is scattered perpendicular to the arrays. That leads to the following remarks: a) The nature of the crystals is irrelevant, only their function of absorbing light is. Any opaque element could be used. b) The light that gets through has to be scattered by some kind of matter to become visible. c) The direction in which it is scattered is determined not by the position of the scattering elements but by the direction the transmitted em waves have been polarized. The last point is what makes the whole a mystery to me. It implies that the scatterers might as well be totally absent for the influence they have on the direction the light is scattered. edit: I have one or two more remarks on the subject, but I have to think (and probably read) more before I place them.

You are making of scattering and polarization an effect caused by the scatterers, or air molecules. But those are the same air molecules that allow unpolarized light to go though them. We only get polarized light with the use of polarizers, be they minerals or technological artifacts like Polaroid filters.

from itself? That is quite a mystery then? Change of direction without external cause? Or rather indifference to external causes? If light, or photons, or electrons, or waves, or whatever they are, fall on a surface, shouldn't they "bounce off" that surface?

How big would the elements need to be to get illuminated on only one side? And how ordered would they need to be to all scatter light in the same direction?

I had exactly the same results as Bragg. That's the whole question isn't it? Gratings can also be said to pass and absorb the light. The question now is what do polarizers do exactly? And how would you describe them, or draw them?

I don't know, ti seems quite obvious that gratings and filters are different things. What makes you think they are the same? Like I said before, I have repeated the experiment.

I must say that is not my experience as I looked down on the beam (that I could not see from aside).

Horizontal and vertical are relative terms. We could look at the setup lying down on one side, and the horizontal and vertical wave would switch places. To be sure, we could also take a picture from different angles. I like the drawing even if it makes me curious. In it, polarizing sheets are not different from gratings. Still, the effects are quite different. That is one difficulty I have with all drawings I came across of polarizing filters. Except Land's, his drawings were incomprehensible to me. Maybe you could help?

I am afraid that would bring us all the way back to the beginning of this discussion. It has also to do with what @Strangesaid: "Light itself is not visible, only when it bounces off something". For us not to see it from one side would mean that somehow a wall has been erected between the illuminated particles and us. Since we can walk around the water tank, that means that one time the beam is contained between two opaque walls that only open at the top, allowing it to be reflected on the mirror. The other time, there is a wall between the mirror and the water and we see the beam only from aside. The polarizations effects we are seeing here are caused either by natural minerals (tourmalin, some form of calcite), or by the Polaroid sheets Land had patented. Maybe that an explanation can be found which would explain how come these minerals or sheets have these effect, but I admit my ignorance on this point. What I have read did not make much sense to me and I would really like to understand. This is irrelevant. Instead of the mirror you can simply have an observer looking down, and it won't matter at what angle he holds his head.

I would certainly hope not. I am not doubting your expertise but simply saying that the theoretical explanation is lacking, at least in the case at hand. I have no reason to doubt calculations made in Optics or Physics in general. Maybe there is a good explanation, but I have not come across it yet.

It sounds like you understand it no more than I do.

With all due respect you are repeating what he said and what can be found in all textbooks. I just do not see how it can be applied to the situation here. How can a beam that is reflected on a mirror not be visible? Even if it scatters vertically it should still be visible from aside. The same way, if it scatters horizontally it should still be visible from above. After all, a square or rectangle or cube can be understood as horizontal planes or vertical planes stacked together.

Okay. I find it very strange but okay. You shine a light through a water tank with water that has been made murky to make light beams visible. With no polarizing filter and the light turned on you can see the beam crossing the tank and reflected on a mirror above. When using a polarizing filter you get to see, depending on how you hold the filter, either the beam through the water, or its reflection on the mirror. Never both at the same time. May I also remind you that we are talking about a lecture of the eminent physicist sir Bragg? I apologize for not reacting sooner. Your remark got lost in the discussion following it. I just want to say that disconnecting polarization and vision still does not explain Bragg's experiment.

I have repeated Bragg's experiment as well as I could. I used a led electric torch much less powerful than the lamp he used, but the contrast was high enough to clearly distinguish between different times when the beam was "invisible" and when visible. I could watch the water tank from all sides except the bottom. In all cases, however the polarizing sheet was set, I could see the beam reflected on the opposite side of the water tank, and on the wall beyond it. The light was therefore not absorbed, certainly not in its totality. So that still leaves the question how the beam could go through the water, not be visible to the eye, and still have a reflection on the mirror, or be seen when looked at from above. This while the beam was each time either visible or invisible from both sides. I must admit that I see very little difference between the diffusion caused by mist and the way the water looks when the beam is polarized. In both cases, it seems like the beam, even if present, is much too "diluted" to leave a clear trace. Like I said, I find light polarization very puzzling, and I hope you will be able to clarify it for me.

I reread the info just to make sure I had not missed anything, but I am afraid that it still does not explain why, for instance, we see the beam through the water in one position of the filter, and its mirror reflection in another position. Say the beam is totally transmitted in one case, then how come we do not see its reflection on the mirror? What could possibly stop this reflection? In the other case, how could there be a mirror reflection without the beam that is being reflected? Maybe you could show me how the rules can be applied to the example in question?

Sir Bragg Sometimes we see the beam, sometimes we do not. At least, not at the same location.

how does that relate to the fact that we can see objects/reflections or not?

I couldn't agree more. Concerning your other remarks. They are also very convincing and I need to review my arguments and maybe views on the matter. I propose to concentrate on the relation vision and polarization. This would solve at least half of the puzzle: we do not need to see that which does not exist anymore. If we continue this line of reasoning, then the fact that we can see polarized light has nothing to do with it being polarized, and we do not need to try and explain how vibrations influence vision. I am not sure that is the right explanation, but this remark seems to imply it.

You are mostly right: "Most animals do not have eyes that are sensitive to polarized light or behavioral activities that require polarized light. Wilhelm Karl von Haidinger (born February 5, 1795, died March 19, 1871) published a paper in 1844 announcing his discovery that the human eye does perceive linearly polarized light. This visual sensation is manifested as two opposing paddle-shaped yellow regions with blue areas orthogonal to the yellow. This pattern, known as Haidinger’s brushes, is best seen when looking at a highly polarized white background." Dennis H. Goldstein, Polarized Light, 2011. Still, what is certain, is that we can see its effects. I find the following quote from the same author very interesting: "light reflecting off an object on the dashboard reflects back to the inner surface of the windshield and then to the driver, and is polarized horizontally. The sunglasses eliminate this image as if by magic." (p.7) [my emphasis] We have all seen how reflections on a window seem to disappear when we are using Polaroid filters or glasses. Even more mysterious are the examples of polarization where the effects are seen with the naked eye, as the example given above by sir Bragg. "

No, I need a lecture neither on vision nor on Polarization, but an explanation of how they can be fitted with each other. Do vision theories support the physical explanation, do they contradict them?

@HallsofIvy I was hoping you would go beyond the textual clarifications and address the issues themselves. Especially the one I formulated in the following lines: I have to admit that I find polarization the most puzzling subject in all of physical optics. It is not so much a matter of understanding, since there is nothing really complicated about the explanations given, but more about how unconvincing they sound to my ears. What I understand of vision, and without being an expert I can say that I have studied it quite thoroughly, I just cannot seem to rime it with the explanations given. I hoped that this forum would help overcome my skepticism.

okay. Do you maybe have a link or reference for me? I would really appreciate it. edit: You will understand that spectrograph's would not really be conclusive. We do not need to go to Mars for that. It is already undeniably proven that different elements show different absorption patterns. What I would find interesting is to see an experiment comparable to that originally done by Newton whereby a ray of sunshine is let through a small opening and then through a prism. That would be the definite proof that the atmosphere and/or the prism play no role in the composition of the spectrum. It would be a little bit like Armstrong's experiment where he dropped a feather and a heavy object on the moon, and they fell at the same speed. That was in fact the first empirical proof of the Galilean and Newtonian concept of gravity outside of earth conditions. So, your "prediction" is certainly understandable. What goes for Earth should be valid on Mars also. Still, conviction is not proof.

Is that a prediction, or have experiments taken place that show these results?

Thank you for the link. And what would you conclude from the comparison between Earth and Mars? Would you say that the size of the particles is determinant, as Raleigh said in the quote above? To be honest, I did not read his other articles (yet) and I do not know if he kept the same assumption throughout his long career. edit: Do you know maybe of any reference of the spectrum through a prism on Mars?