# Why, in Bragg's law, is the word “diffraction” used? When Bragg's law is explained, the word "diffraction" is used. But, as far as I see the picture above which is often added to the explanation, it seems what happens is not diffraction but just reflection.

I believe diffraction and reflection is different. Why is the word "diffracion" used?

Thank you in advance.

## 1 Answer

In Fraunhofer diffraction, every points at the aperture (the opening) are regarded as secondary sources. Mathematically, the wave fronts are undergone Fourier transform of a step function over the aperture.

We extend the idea to a crystal lattice. Waves "diffracted" from each particles (atoms, ions, molecules and so on) are regarded as point sources (at least to a good approximation). More precisely, the particles scatter the incoming waves with regular phase differences. The diffracted waves are hence the Fourier transforms of periodic delta functions (layer by layer with constant phase shift).

• Thank you. So the initial X-ray is bent by hitting particles of a lattice and thus the phenomenon is called "diffraction"? I thought "diffraction" is a phenomenon of waves wrapping around an abstacle. (At least, diffraction is taught like this in Japan.) But what happens now is redirection of the initial waves. Is this phenomenon also called "diffraction"? Also, is the diffracted wave 3D spherical wave? (though the initial wave is X-"ray", or "beam") – ynn May 12 '17 at 11:52
• The scattered wave fronts are spherical (at least up to first order). In usual practice, diffraction is defined as superposition of wave front on its own whereas interference as superposition of different wave fronts. I recommend not to be too stuck on the terminology, it's not as certain as mathematics. – Ng Chung Tak May 12 '17 at 12:30
• See the discussions here and don't miss out another point of view here. – Ng Chung Tak May 12 '17 at 12:30
• Thank you very much. Your detailed answer really helps me a lot. – ynn May 12 '17 at 12:42