What is "bragg's law"

n. (context physics singulare tantum English) equates the angle $(theta)$ between the incident and scattered ray to the spacing $(d)$ between the crystal planes and the wavelength $(lambda)$ of the radiation - $nlambda=2dsin(theta)\; ,$

In physics, Bragg's Law (or "Wulff-Bragg's condition" in post-Soviet countries, a special case of Laue diffraction) gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave. The movement of these charges re-radiates waves with the same frequency, blurred slightly due to a variety of effects; this phenomenon is known as Rayleigh scattering (or elastic scattering). The scattered waves can themselves be scattered but this secondary scattering is assumed to be negligible.

A similar process occurs upon scattering neutron waves from the nuclei or by a coherent spin interaction with an unpaired electron. These re-emitted wave fields interfere with each other either constructively or destructively (overlapping waves either add up together to produce stronger peaks or are subtracted from each other to some degree), producing a diffraction pattern on a detector or film. The resulting wave interference pattern is the basis of diffraction analysis. This analysis is called Bragg diffraction.