Question

575 www.allen.ac.in
KOTA (RAJASTHAN)
CAREER INSTITUTE
3 ALLEN"
2
3.
A parallel beam of monochromatic light of wave length 500 nm is incident norma
surface. The power through any cross-section of the beam is 10 W. Find
(a) Number of photon absorbed by the surface per second.
(b) The force exerted by light beam on the surface.
Liab aalau of photon hac arcator anorgueither red or plot 2​

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