Bragg's law

Etymology
Bragg diffraction was first proposed by Lawrence Bragg and his father William Henry Bragg in 1913.

Proper noun

 * 1)  A law that 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 - $$n\lambda=2d\sin(\theta) \,$$