I have been trying to research this and my understanding seems to be flawed. From what I have gathered from light sails and other sources: 1) The frequency of reflected light is the same as the incident light. 2 ) The photon imparts a kick to the reflected surface, Transfers momentum. This seems to paint a picture of a photon reflecting between two surfaces without losing momentum/frequency while imparting momentum at every reflection. Clearly I am missing something here.