Bragg's law

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The fundamental law of x-ray crystallography, nλ = 2dsinθ, where n is an integer, λ is the wavelength of a beam of x-rays incident on a crystal with lattice planes separated by distance d, and θ is the Bragg angle.

Wiktionary

1. n. physics, singular only equates the angle between the incident and scattered ray to the spacing between the crystal planes and the wavelength of the radiation -