Why does X-Ray diffraction work without a coherent source ?

 

I have an idea about the answer. Maybe the photons do not need to interact with each other. Maybe diffraction also works with a single photon which interacts with itself. However Bragg's equation is always described with multiple photons and different path lengths in the crystal...

Moreover, I bthought that X-rays could be used to observed periodicities much larger than the wavelength. Is it possible that a single atom give information about such distances ?

 

Where is the mistake ?

Coherence is a little more subtle than this. You can make sunlight interfere, even though it's not a coherent source like a laser. The issue is, how coherent is it — it's not a binary condition.

 

Coherence is two waves having a definite phase relation, and this can be maintained for some length of time, which means it will be maintained over some length of travel. Laser light can be coherent over kilometers of travel, but as long as the light is coherent over the length of travel where you have a path difference in your interferometer, you will get interference.

 

There's an excellent primer on coherence here: http://skullsinthestars.com/2008/09/03/optics-basics-coherence/