We know that, when photons of light is incident on a surface, the change in momentum of photons takes place resulting in a force exertion by the photons on the surface. Let light of power W is incident on a surface.
Each photon carries energy hv and,momentum = hv/c .....(i)
Energy incident in unit time =W(by the definition of power) But, power is given by
Force exerted by photons on a perfectly absorbing surface :
In this case, change of momentum suffered by each photon = hv/c
using equation(iii), we get
force = (W/hv) x (hv/c) = W/c
thus, F = W/c for a perfectly absorbing surface.
Force exerted by photons on a perfectly reflecting surface:
In this case, change of momentum suffered by
Force exerted by photons on a surface of reflection coefficient ρ:
In this case momentum of incident photon = hv/c,
Momentum of reflected photon = ρ hv/c
