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Refraction: I want a qualitative Quantum Mechanical explanation of why do we see light rays -in the classical picture- bend when light goes from one medium to another. I read that it is due to conservation of energy and momentum, but haven't found an explanation about the reason of the change of angle.
Reflection: again a qualitative explanation of why the angle of incidence is equal to the angle of reflection.

EDIT 1: what I am looking for is really an explanation about what interactions are the photons going through(like absorption and emission by molecules for example) that when we take all the photon's interactions into account give us the macroscopic picture of how light reflects and refracts.

EDIT 2: I really want an answer containing the reason for why we get the angle of incidence is equal to the angle of reflection and why the angle if refraction is as it is.

glS
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TheQuantumMan
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    There are no light rays in QM, so you can't really get an explanation for "that". What you can do is to couple a free em-wave state to a crystal lattice and recover that the direction of the wave vector of the quasi-particle state that exists inside the medium follows the known laws for refraction. Would that satisfy your request conceptually? I do believe that we had this question multiple times already, though... somebody will certainly dig up the links for you. – CuriousOne Nov 01 '15 at 15:48
  • I have edited just when you posted the comment! Well, what I want really is what happens at a microscopic level that gives us the macroscopic picture of light that we have(the usual like angle of incidence is equal to angle of reflection and so on) – TheQuantumMan Nov 01 '15 at 15:51
  • At the microscopic level the free em-state becomes a quasi-particle state. The results are, as far as I know, basically identical to semi-classical theory, unless we start looking at things like the interaction of a laser field with e.g. a bunch of ultracold atoms. – CuriousOne Nov 01 '15 at 15:54
  • To say the truth, I am looking something along the lines of photon absorption and re-emition and conservation of energy and momentum, as these are the usual things that books are concerned with(and are at my level). I have the nearly full picture with the components that I mentioned but the reason of the angle of the two phenomena is what is missing. – TheQuantumMan Nov 01 '15 at 15:56
  • Then you are looking at the wrong thing, since individual photons are completely irrelevant for physics at the level of ray optics. Ray optics follows from wave optics and there are no quantum mechanical effects needed to recover it from "first principles". QM interaction of light with solids doesn't have to conserve either momentum or energy (and in those cases that are of practical interest it does neither), so, again, you are missing virtually 100% of the interesting physics with the question by sticking to a theoretical border case that doesn't even happen. – CuriousOne Nov 01 '15 at 15:58
  • I took a crack at a similar question here. Check and see if that helps. – garyp Nov 01 '15 at 16:00
  • No, I am not looking about explanation with individual photons. I know that the observed macroscopical effects are due to the net result from the photons, but I just don't know the underlying mechanism that those photons go through in order to get the macroscopic picture. – TheQuantumMan Nov 01 '15 at 16:01
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    Possible duplicates: https://physics.stackexchange.com/q/2041/2451 , https://physics.stackexchange.com/q/6428/2451 , https://physics.stackexchange.com/q/10301/2451 , https://physics.stackexchange.com/q/83105/2451 and links therein. – Qmechanic Nov 01 '15 at 16:06
  • @garyp yes it helps, but I want to go the extra mile that the accepted answer just about shies away from! But yeah, that is the direction of the answer I am looking for. – TheQuantumMan Nov 01 '15 at 16:06
  • So my question is... if you go the extra mile... just how are you selecting the few QM solutions that have the properties that you want from the ones that don't... which is basically all of them? The interaction of solids with light is extremely rich and all you are asking for is the most boring case. QM doesn't give you just the most boring case, it gives you everything, at once, unless you start throwing additional ad-hoc assumptions into your "derivation". – CuriousOne Nov 01 '15 at 16:15
  • Final comment (perhaps). I note your phrase "net result from the photons". I'm not sure what you mean, but if you are thinking of photons as "particles", little marbles, you are doomed to failure. Better to think of them as excitations of the EM field. – garyp Nov 01 '15 at 16:23
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    From this, you should really rephrase your question to account from 1: what you have learn since 2: what would help people understanding what you already know and what you seek at. – Fabrice NEYRET Nov 01 '15 at 16:29
  • Guys, I am really uninitiated as far as QM goes. I was reading optics of Hecht where he explains the properties of light and gives a simple explanation of what happens microscopically with photons and how we get the macroscopic picture. Its simple stuff really. I don't choose anything. I just follow the pattenr of the book. But the book does not really explain what is the reason of the angle of reflection and refraction and I just want to qualitatively find out the reason about this. – TheQuantumMan Nov 01 '15 at 16:32
  • Why not read this? Should be pretty cheap. https://www.goodreads.com/book/show/5552.QED Also the best thing I ever read. – Řídící Nov 01 '15 at 17:46
  • To reopen this post (v6) consider to focus question. In particular align title and main body. Tip: Let's not have posts look like revision histories. – Qmechanic Aug 27 '19 at 08:12

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How a classical electromagnetic wave emerges from innumerable photons can be seen in this blog entry. It is not simple, one needs quantum field theory to start with. One should get the interaction of a single photon with a crystal lattice , and one can get a quantum mechanical solution, which will give the probability of the photon to scatter or go through the crystal. Then one has to use the logic/math, outlined in the blog link above, to see how the classical beam with its diffraction would emerge

EDIT: what I am looking for is really an explanation about what interactions are the photons going through(like absorption and emission by molecules for example) that when we take all the photon's interactions into account give us the macroscopic picture of how light reflects and refracts.

Photons can interact with matter by

a) elastic scattering : only the angle changes and not the energy

b)inelastic scattering with the field of the matter they hit: in this case the frequency changes and thus the color.

c)absorption by atomic and molecular layers: in this case the photon disappears and no longer contributes to the light beam. The atom may de-excite and an equal frequency photon come out, but it will not longer be coherent with the light beam because the direction of emission will be different than the direction of the macroscopic beam.

So in reflection one can handwave of the individual photons scattering elastically and keeping the phases between them, and thus the images can be reflected.

In refraction though the quantum mechanical solutions have to come in so as to show that the scattered photons keep a coherence, and I cannot see how without solving for a specific lattice and summing up the individual photons one can handwave an index of refraction. See also the answer by Marek here. and this link here.

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