Question

A photocell is illuminated by a small bright source placed $1m$ away. When the same source of light is placed $\frac{1}{2}m$ away, the number of electrons emitted by photo cathode would

• 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 $2$

Answer 1

The correct option is B increase by a factor of $4$

As we know,

Number of photoelectrons $\propto \frac{1}{(Distance{)}^2}$

New Number of photoelectrons $\propto \frac{1}{(\frac{Distance}{2}{)}^2}$

New Number of photoelectrons $=4\times$ Number of photoelectrons

hence,

the number of electrons emitted by photocathode would increase by a factor of 4.