Bragg's law describes the reflection of a monochromatic x-ray beam by two parallel crystalline planes separated by a distance . The difference in path lengths is equal to and this determines a necessary condition for constructive interference from a family of parallel planes. The Bragg condition enables spacings between many different sets of crystalline planes to be determined.

The graphic is schematic: for simplicity, it is assumed that for every choice of angle and frequency, the lattice distance implicitly changes to exhibit constructive interference. Otherwise, the X-rays interfere destructively and there is no diffraction pattern.