Of course, if the waves add up in phase in some directions, they have to cancel out in a lot of other directions. That is why the diffraction pattern from a crystal is an array of spots. There are a number of potential bottlenecks in determining a crystal structure, but growing a useful crystal can be the most serious one. If you can't collect any data or only bad data, you won't be able to solve a structure. So we'll spend the next lecture looking at the theory and practice of crystallisation.
Diffraction Think of the electrons in a crystal as emitters of little waves.
Which they do depends on the direction of the incoming and outgoing waves and the positions of the electrons relative to each other. The total path from the source to the detector will determine what happens. If the difference in path taken via one electron and another is a multiple of the wavelength, then the waves will scatter in phase and their amplitudes will add up. If it's a half-integral multiple of the pathlength, they will scatter exactly out of phase and cancel out. The conditions for scattering in phase can be summed up quite neatly if you think of the waves being reflected off of planes passing through the atoms.