I have some questions about solar sails. Basically, what happens to the photon? I am under the impression that the "sail" is a giant mirror that reflects photons striking it. Since photons have momentum, they impart some of this momentum to this mirror. Is this correct? But that must mean that each photon loses momentum/energy. Does this just mean that the reflected photons will always be of a lower frequency? Does anyone know what determines during a photon collision with another particle how much momentum is transfered as opposed to other collision effects? Thanks.