A useful explanation of von Laue's experiment was formulated by W.L. Bragg in Cambridge, also in 1912. Bragg considered the diffractions to arise from “reflections” of separate X-ray waves from parallel planes of electron density. If the perpendicular distance between the planes is d_hkl then diffraction will occur at the Bragg angle θ because at this angle the distances traveled by the two waves differ by one wavelength and they are said to be “in phase” resulting in constructive interference:

The question now is, “What are those planes of electron densities?” in Bragg's Law.