A very common image for the concept of Bragg Scattering is the following:

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If my interpretation is indeed correct, it shows an incident plane wave, and with it, two rays of light. One happens to bounce off of an atom of the crystal, and the other doesn't traveling down to the next spacing level and bouncing off that. Bragg's Law, if we assume this all happens, makes perfect sense to me, however, I've always had one question:

If the momentum of photons cannot be measured like we do with classical objects, how can we simply say that one ray of light hits the top layer of atoms and one ray hits the bottom layer as if they move classically?

Those arrows, I'm assuming, are.. some kind of depiction of a ray of light or a photon.. although it's supposed to represent one plane wave. It just looks weird to me. The plane wave has constituent arrows of light that are big enough to interact with different parts of the crystal's atomic structure?


Rays, of course, don't exist. Each one of those arrows is a representation of the same plane wave. If you like, you could think of each arrow as a thin pencil chosen (but not separated) from the plane wave. The plane wave, of course, fills all of space.

A way to think about this is to imagine that the plane wave is partially reflected at the first surface. This is represented by the upper ray. Some of the energy of the plane wave reaches the second layer $\ldots$ represented by the lower arrow.


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