Bragg's law

the law that the intensity of a crystal reflection of an x-ray is a function of the angle (Bragg angle ) that is the complement of the angle of incidence of the x-ray.

the principle that when a beam of X-rays of wavelength λ enters a crystal, the maximum intensity of the reflected ray occurs when sin θ = n λ/2 d, where θ is the complement of the angle of incidence, n is a whole number, and d is the distance between layers of atoms