Hi, I am just trying to understand the basis of radiation pressure. I understand radiation pressure due to absorption, but I am having a hard time understanding the radiation pressure due to reflection. From what I understand there will be an incoming photon with momentum p = E/c. The normal momentum will be imparted into the reflecting surface upon impact, resulting in a transfer of energy of Ef/c * cos(a) for incident angle a. Now the photon will also be reflected in a random direction. I think that there should be an integral over the solid angle to capture all possible reflection angles, but I am not sure what to do exactly. I know Wiki says Preflec=2Ef/c * cos2a for an incident angle a. I am not sure where the cos squared comes from.