Question

A 5 watt source emits monochromatic light of wavelength 5000 Å. When placed 0.5 m away, it liberates photo electrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m, the number of photo electrons liberated will be reduced by a factor of

(a) 8

(b) 16

(c) 2

(d) 4

Answer 1

The correct option is (d) 4.

Explanation:

For a light source of power P watt, the intensity at a distance d is given by

where we assume light to spread out uniformly in all directions i.e. it is a spherical source

In a photoelectric emission, the number of photo electrons liberated per second from a photosensitive metallic surface is proportional to the intensity of the light. When a intensity of source is reduced by a factor of four, the number of photo electrons is also reduced by a factor of 4.