The same phenomenon, considered from a different viewpoint, is described by a beam of particles of momentum p incident at angle θ upon the same array of crystal atomic planes. It is supposed that a collective of n such atomic planes reflects the particle, transferring to it a momentum nP, where P is a momentum characteristic of the reflecting planes, in the direction perpendicular to them. The reflection is elastic, with negligible transfer of kinetic energy, because the crystal is massive. The initial momentum of the particle in the direction perpendicular to the reflecting planes was p sin θ. For reflection, the change of momentum of the particle in that direction must be 2 p sin θ. Consequently,