A Crookes radiometer does not demonstrate radiation pressure. Instead, it demonstrates the pressure of hot air that is heated by the energy that light carries to a black surface.

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the energy flux density divided by the speed of light. If the radiation is totally reflected, the radiation pressure is doubled. This happens because the waves that make up electromagnetic radiation carry momentum with them and pass that momentum on when they reach a surface.