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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 Jun 24 '15 at 19:55
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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 Jun 24 '15 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 Jun 24 '15 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 Jun 24 '15 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 Jun 25 '15 at 11:26

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