Question

A parallel beam of monochromatic light of wavelength 500 NM is incident normally on one perfectly absorbing surface the power through any cross section is 100 what
Find the the number of photon absorbed per second by the absorber
Find the force exerted by the beam on the absorber

Answer 1

I) 2.5 * 10^10

2) 3.33 * 10^(-7) N

Explanation

no of photons : n

energy of a photon E = hv

energy of n photons E = nhv

frequency = c/d

Power = Energy/time

100 = n 6.63^(-34) * 3*10^8/500*10^(-9)

n = 100/3.978*10^(-9)

0.2513 * 10^11 = 2.5 * 10^10

no of photons = 2.5 * 10^10

ii) Power = Force * Speed

Force = P/S

Force = 100/3*10^8

= 0.333 * 10^(-6)

Force = 3.33 * 10^(-7) N