Bragg's Law, formulated by Sir William Henry Bragg and his son Sir William Lawrence Bragg in 1913, describes the relationship between the wavelength of incident X-rays and the crystal lattice spacing. The law is mathematically expressed as: nλ = 2d sinθ where n is an integer (the order of the diffraction), λ is the wavelength of the X-rays, d is the distance between crystal planes, and θ is the angle of incidence.