The Bragg law
Diffraction is an interference process between waves coherently scattered by obstacles. Diffraction of slow neutrons makes it possible to observe matter at the atomic or molecular scale, giving a snapshot (or an average view) of its internal structure. Diffraction by crystalline materials is governed by the Bragg law:
nλ = 2 d sinθ
It says that if the distance between atomic planes is d and the incident angle of the wave is θ, then the beams reflected by various planes only interfere in a constructive manner when the difference between their optical paths (shown in blue) is equal to a multiple of the wavelength λ.