(a) Each photon carries energy and momentum. Hence when photons of light is incident on a surface, the light is either absorbed or reflected or both is done by the surface.
(b) change in momentum of light takes place, which causes impulse or force on the surface resulting in a pressure called radiation pressure.
Let light of intensity I is incident on a surface. Each photon carries energy hv and momentum= hv/c.
Energy incident on unit area in unit time = I(by the definition of intensity). Number of photons incident on unit area in unit time.
Radiation pressure for perfectly absorbing surface :
In this case, change of momentum suffered by each photon = hv/c.
Using the equation(iii), we get
Radiation pressure = 1/hv x hv/c = 1/c
∴ Radiation pressure = I/c for a perfectly absorbing surface.
Radiation pressure for a perfectly reflecting surface :
In this case, change in momentum of each photon = hv/c - (- hv/c)
Hence, using the equation (iii), we get
Radiation pressure = (I/hv) x (2hv/c) = 2I/c
∴ Radiation pressure = 2I/c for a perfectly reflecting surface.
Radiation pressure for a surface of reflection coefficient (ρ) :
In this case, Momentum of incident photon = hv/c
