Question

If a parallel beam of light of energy density U falls normally on an object and totally absorbed what is the pressure exerted on body

Answer 1

photon momentum equals to

$$p_p = \frac{\epsilon}{c},$$

therefore by unit area per unit of time,

$$\frac{U}{c}$$

momentum is absorbed; this value equals to pressure p, that is

$$p =\frac{U}{c}$$