Question

A parallel beam of monochromatic light of wavelength 500 nm is incident normally on a perfectly absorbing surface the power through any cross section of the beam is 10 w find

Answer 1

(a) Energy of each photon is

E = hc /  

 = 1242 eV – nm/ 500 nm

 = 2.48 eV

In one second 10 J energy passes through any cross section of the beam,

Thus the no. of photons crossing a cross section is = 

 n = 10 J / 2.48 eV

n = 2.52 * 1019

 

(b)

total momentum of all the photons = 

 p = nh/  = nh / c = 10 J / 3*108

 = 3.33 * 10 –8 N-sec