Bragg's law (Sir W.L. Bragg; 1912)
While a beam of x-rays strikes at crystal surface wherein the layers of atoms or ions are separated regularly, the maximum intensity of the reflected ray takes place while the complement of the angle of incidence, theta, the wavelength of x-rays, lambda, and the distance among layers of atoms or ions, d, are associated by the equation
2 d sin theta = n lambda,
Where n is an integer.