Bragg diffraction occurs on the atomic crystal lattice. It conserves the particle energy and thus is called elastic scattering. The wave numbers of the final and incident particles, k f and k i , respectively, are equal. Just the direction changes by a reciprocal lattice vector G → → = Q → → = k → → f − − k → → i with the relation to the lattice spacing G = 2 π π / d . As momentum is conserved, the transfer of momentum occurs to crystal momentum.