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It seems like a very common model that is taught in schools for crystal diffraction is that there is an incident plane wave of light on a crystal lattice, and each atom in the lattice scatters the light, and the light interferes.

But it seems like the scattering part itself isn't diffraction. For example, this wikipedia page for Thomson scattering says that it's responsible for x ray crystallography, but the word 'diffraction' doesn't appear once on the page. In that page, they present the scattering as the usual dipole radiation thing, which is not diffraction, right?

So is diffraction actually present here or is it a misnomer? It seems like it could be, with the x-rays 'bending' around the atoms, but it seems like the dipole radiation takes care of that instead.

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    $\begingroup$ You are running into the usual trouble with finding a definition for "diffraction". It is a arguable position that there is no such definition. But the people who use that technique refer to it as diffraction, and you will also here people discuss "diffraction scattering" at time. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jun 24, 2015 at 19:55
  • $\begingroup$ The dipole radiation thing is scattering and not diffraction, i.e. it behaves more like particles than waves. Diffraction involves an aperture or edge and is a result of wave behaviour ... and we still have waves after diffraction. Diffraction patterns are another special case on their own ... also based on wave behaviour. (Note that water waves thru a single aperture diffract but do NOT interfere ..... but light does interfere in a single slit). $\endgroup$
    – PhysicsDave
    Commented Jun 27, 2022 at 1:16

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The atoms in the lattice can be thought of as coherent re-radiators of the incident photons. This is not unlike the scenario we have in a double slit experiment, where a Huygens construction of the wave front considers each point in the slit as a radiation source.

So it might be "opinion" but I think that diffraction is an appropriate word to use.

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  • $\begingroup$ Thanks, but diffraction isn't actually re-radiation, is it? The Huygens wavelet principle isn't actually physical, just a nice model, I thought. Whereas dipole radiation is very real. $\endgroup$
    – YungHummmma
    Commented Jun 24, 2015 at 16:35
  • $\begingroup$ This is why I marked my comment as "opinion" and said that I thought the use of the word was appropriate. What is your definition of diffraction? $\endgroup$
    – Floris
    Commented Jun 24, 2015 at 16:36
  • $\begingroup$ I'm not sure... That's partly what I'm confused about and trying to figure out. Wikipedia says "Diffraction refers to various phenomena which occur when a wave encounters an obstacle or a slit", but that doesn't really define the phenomena it's talking about. The thing I've always heard is how light "bends" around an object even in a vacuum. $\endgroup$
    – YungHummmma
    Commented Jun 24, 2015 at 17:06
  • $\begingroup$ Even in the case of "classical standard" edge diffraction you can view diffraction as being the result of interference of the radiation of the induced edge current and the incident wave. You cannot have EM diffraction without an obstacle and interference of the primary and secondary waves that are the result of the obstacle being there. It is true that an idealized spatial restriction may act as a bandpass filter on the Fourier components resulting in Gibbs phenomenon, etc. and hence appears to induce diffraction but to act as a filter it must be able to interact with the EM field. $\endgroup$
    – hyportnex
    Commented Jun 25, 2015 at 11:26
  • $\begingroup$ @Floris We say that diffraction happens when X-rays hit some crystal. Does refraction and reflection also take place? Can we say that all these phenomena, reflection, refraction and diffraction are manifestations of light being scattered from the atoms (or molecules) of the material? $\endgroup$
    – Anton
    Commented May 29, 2022 at 13:55
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In my opinion there is scattering and there is diffracted and it all begins at the edges. Light scatters off and away from an edge but also can diffracts around and behind an edge. A slit has two edges. Single slit interference happens where the scattered light is out of phase with the diffracted light from those two edges.

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