Question

When a point source of monochromatic light is at a distance of 0.2m from a photoelectric cell, the cut-off voltage and the saturation current are 0.6 volt and 18 mA respectively. If the same source is placed 0.6 away from the photoelectric cell, then

• A. The stopping potential will be 0.2 V
• B. The stopping potential will be 0.6 V
• C. The saturation current will be 6 mA
• D. The saturation current will be 18 mA
  

Answer 1

The correct option is B The stopping potential will be 0.6 V

Photoelectric current (i) $\propto$ Intensity $\propto {\frac{1}{\left(distance\right)}}^2$. If distance becomes 0.6 m (i.e. three times) so current becomes $\frac{1}{9}$ times i.e. 2mA.
Also stopping potential is independent of intensity i.e. it remains 0.6 V.