it makes sense that with an inelastic scattering process, a wave approaches a crystal, and then some energy is imparted to the phonons, so that the outgoing wave has a different frequency from the incoming wave. However, with an elastic scattering process, the frequency is unchanged-- how does the crystal not affect the wave? What exactly is happening? I don't understand the precise steps of what is going on, that causes an elastic vs inelastic result. Thanks!