I took a course on protein crystallography last year and there's one thing I couldn't figure out then, and still can't figure out now. My understanding of Bragg's law hinges on the fact that in-phase scattered waves constructively interfere, and the requirement to be in-phase is met only when the extra distance traveled is an integral multiple of the photon's wavelength. With this premise and a diagram of two atoms scattering a photon, it becomes apparent that nλ = 2dsin(θ). What I don't understand is why only maximal-intensity waves are observed. Unless the two waves are π radians out of phase, there should still be some constructive interference that gives rise to amplitude at the detector plate. Therefore, there should be some intensity at almost any value of θ. The wikipedia page on Bragg's law acknowledges this fact: Unfortunately, I don't see a clear and unbroken line of reasoning in the explanation. So many atomic planes means more complex interference... why is that guaranteed to produce mostly destructive interference? For help, I turned to Gale Rhodes' Crystallography made Crystal Clear. He explains it as follows: This is starting to approach an answer but I don't think it's complete. Why must there exist some parallel plane that produces a 180 degree phase shift? Why do such cancelling planes not exist for the set of planes that comply with Bragg's equation? Thanks for any help!