question_answer

A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed 0.5m away, the number of electrons emitted by photocathode would     AIEEE  Solved  Paper-2005

A) decrease by a factor of 4

B) increase by a factor of 4

C) decrease by a factor of 2

D) increase by a factor of 2

Correct Answer: B

Solution :

The ratio of intensities, \[\frac{{{l}_{2}}}{{{l}_{1}}}=\frac{{{({{r}_{1}})}^{2}}}{{{({{r}_{2}})}^{2}}}\]                               where,distance from source. \[\frac{{{l}_{2}}}{{{l}_{1}}}=\frac{{{(1)}^{2}}}{{{\left( \frac{1}{2} \right)}^{2}}}\] \[{{l}_{2}}=4\,{{l}_{1}}\] Now, since number of electrons emitted per second is directly proportional to intensity, so number of electrons emitted by photocathode would increase by a factor of 4.