1. The problem statement, all variables and given/known data Consider a photon colliding with an electron and scattered directly backward. If the electron is initially at rest and the incident photon has frequency f, determine the frequency of the back-scattered photon 2. Relevant equations 3. The attempt at a solution Since the photon has no rest mass, then it should always move in speed of light. Therefore the frequency before and after the collision must be the same.