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.

For adequate choice of angle, phase and frequencies, the lattice distance exhibit constructive interference, otherwise, the X-rays interfere destructively and there is no diffraction pattern (resulting wave in red).