Question

A caesium cell with a steady potential difference of 90 volt across it, is illuminated by a small light placed one meter away. When the same light is placed to two meter away, the electrons crossing the photo cell, will 

(1) each carry one quarter of their previous energy 

(2) each carry one quarter of their previous momentum 

(3) are half as numerous

(4) one quarter as numerous

Answer 1

Correct option : (4) one quarter as numerous

Explanation :

Since the intensity of light varies inversely as the square of the distance; increasing the distance twice of the previous value the intensity of light incident on photocell will be reduced by a factor of 4. Now the number of electrons emitted from a photo surface is directly proportional to the intensity of light, the number of photo electrons emitted will decrease by a factor of 4. Hence they will be one quarter as numerous.