Bragg's law - Whenever a beam of x-rays strikes a crystal surface in which the layers of ions or atoms are often separated, the maximum intensity of the reflected ray takes place when the complement of the angle of incidence, theta (θ), the wavelength of the x-rays, lambda, and the distance between the layers of atoms or ions, d, are related by the equation by:
2 d sin theta = n lambda,
Here n is an integer.