Interference or diffraction? Suppose I have $N$ slits separated by a distance $a$. We are sending a light source of wavelength comparable to the slit width. Then which of the effect should dominate: Interference or diffraction?
The doubt arose because, in the case of the 'X-ray diffraction in a solid-state, we are considering the diffraction of waves only. Why we are not concerned about the interference phenomena? Considering a linear chain of atoms separated by a distance $a$, which phenomena should dominate, and what is the condition for the dominance of interference or diffraction phenomena?
 A: You are confusing different processes, interference is when multiple waves interacts between each other amplifying or canceling each other out. Now how these waves comes into interacting points, is a different process, i.e. beam splitter, thin-film interference, diffraction and others.
So at first something must "push" waves together for them to meet each other, and then they interacts in interference patterns. Classical example,- you bump pair of pebbles into lake creating two dot-sources of pressure waves, which when meets starts to interfere.
A: So if you go back to Young's double slit experiment (YDSE) with slit width $a$ separated by distance $d$: you get interference. As $d\rightarrow 0$, you get diffraction from a slit of width $2a$.
What's the difference? Diffraction through a slit occurs because you don't know if the particle went through the left side of the slit or the right side. This doesn't seem to get noticed until the L and R sides have physical barrier between them.
If the two slits are one: it's diffraction, if you separate the slits: it's interference. If you add a bunch of slits: it's called a diffraction grating. Basically: the name doesn't matter, and the observed pattern is the Fourier transform of the aperture, regardless.
