Question

Light rays with an energy flux of 18 W/cm^2 fall on a non-reflecting surface at normal incidence. If surface has an area of 20 cm^2, find the average force exerted on the surface during 30 minutes.

Answer 1

Quantum physics.
       L = Luminescence,  A = surface area        t = time duration
       N = number of photons in light rays.
        λ = wavelength    ν = frequency  of photon as a wave
         c = speed of light        p = momentum of photon particle

Energy in light rays:  U = L * A * t

Energy in photons : (De Broglie's equation)

      U = N h v = N h c / λ                 λ = h / p
So  U = N h c p / h = N p c
   =>   N p = L A t / c
   => N p / t = L A / c = 18 * 400 / 3 * 10^8 = 2.4 *10^-5  N  =2.4 dynes

Momentum transferred from photons to the body with  given surface area per unit time 
    F  = force exerted on the surface area
    F  =  (N p) / t = 2.4 dynes

We can calculate the pressure exerted on the surface area
   P = F / A
       = 2.4 dynes/ 20 cm²
       = 0.12 dynes/ cm²