Question

When a monochromatic source of light is at a distance of 0.2 m from a photocell, the stopping potential (cut off voltage) and the saturation current are found to be 1 volt and 27 mA respectively. If the same source is placed at a distance 0.6 m from the cell then

• A. the stopping potential will be 0.25 V and current will be 27 mA
• B. the stopping potential will be 1 volt and current will be 3 mA
• C. the stopping potential will be 1 volt and current will be 9 mA
• D. the stopping potential will be 1 volt and current will be 12 mA

Answer 1

The correct option is B the stopping potential will be 1 volt and current will be 3 mA

Stopping potential remains the same as it is independent of intensity of light.
Saturation current $\propto$ Intensity
$i_1\propto I_1\propto \frac{1}{r_1^2}$
$i_2\propto I_2\propto \frac{1}{r_2^2}$
$\Rightarrow \frac{i_2}{i_1}={\left(\frac{r_1}{r_2}\right)}^2$
$={\left(\frac{0.2}{0.6}\right)}^2$
$i_2=\frac{1}{9}\times i_1$
$=\frac{27}{9}$
$=3mA$