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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.

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1 Answers1

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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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