Bragg's law

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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 -