Ive seen similar questions elsewhere and people say that you can consider a ray that comes not from the atom directly beneath, but an atom beneath and to the left. That ray will coincide with some ray. I don't like that interpretation because it seems contrived. Also that interpretation does not seem to work for the case of low-energy electron diffraction on a crystal where only the first layer of atoms is touched. In which case the equation ##dsin\phi=n\lambda## and not Bragg's law works. So my question is why does Braggs law work and the e- diffraction formula work if parallel rays never meet? I'm sorry if my question is too dumb. Here's what I mean by the way (with the first paragraph): - x - x - - Consider those two atoms with rays coming out towards the upperright corner of your screen. THe idea is that those rays do in fact coincide.