Hi, When radiation pressure exerts a force on an object causing it to move, where does the object's kinetic energy come from, and how? E.g. consider a perfectly reflecting solid block in vacuum. Its face is illuminated by light. From conservation of momentum, the reflection of the light from the block's surface causes the radiation force, which accelerates the block. The block has gained KE, but it can't be absorbing the photons, since it is perfectly reflective. Does this mean that the reflected photons are actually lower energy than the incident photons? But how? Some kind of Doppler shifting going on? Any help is much appreciated!