Radiation Pressure
Pressure is Force/Area, and Force is change in momentum with respect to time. Hence, electromagnetic radiation should exert a pressure on objects when it hits them.
Radiation Pressure = F/A =
(dp/dt) / A = [d(Energy/speed)/dt] / A =
[Power/speed] / A = [Power/Area] / speed = S/c.
If the radiation reflects, then the momentum is twice and so the radiation pressure is also twice.