In undergraduate solid state physics, derivations of Bragg scattering are often short and scarce in detail, often resorting to virtual "lattice planes" (which I do not find convincing) or Huygens's Principle, like in this image from Wikipedia:

Huygens's Principle applied to lattice scatterrers

This image has raised a question in my head related to whether it is correct. I think the question boils down to whether an incoherent source, say, a light bulb with a collimator, will produce Bragg peaks, and why?

Researchgate (which I am taking very critically because I often find questionable answers there) answers explicitly that coherence is not required:

In most cases of routine XRD experiments on macroscopic (or even microscopic) samples, you don't need coherence, you need just to have a relative monochromaticity (delta lamda almost zero) of X-rays and with relative high directionality (focusing of x-ray to avoid scattering).

An answer to a related question on SE talks about how it is actually the same source wave that later interferes with itself, so there is a well-defined phase difference even if the source is incoherent. But in the case of Huygens's Principle picture that does not add up, as that picture requires two photons (in case of X-ray scattering) to excite two scatterers, and so the two have to be in phase before scattering, requiring a coherent source. This upholds that incoherent source is okay, but does not help understanding Bragg scattering from Huygens's Principle.

In this pictorial idea with Huygens's Principle, it is implied that the "diffraction" is actually an interference effect of re-emitting. This question on SE about thin films speaks of an explanation from an undergraduate book that Laue diffraction is actually an interference effect from multiple scatterers in the lattice. However, for interference, I would expect a coherent source is strictly neccessary for discernible interference pattern, which stands in contradiction to ResearchGate and interference from thin films.

So I repeat the question, will an incoherent source produce Bragg peaks? Can I understand this though Huygens's Principle?



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