For diffraction to occur in crystal, Bragg equation must be satisfied and h,k,l must be of certain combination. I would like to ask 1. How to distinguish a second order diffraction of (100) and first order diffraction of (200)? (they occurs at the same angle.) 2. How can a (200) plane diffraction of simple cubic possible? It seems that there is no atom from this plane because atom only at the four corners of a unit cell.