Question

When a monochromatic point source of light is at a distance of 0.2 m from a photo-electric cell, the cut-off voltage and the saturation current are, respectively, 0.6 V and 18.0 mA. If the same source is placed 0.6 m away from the photo-electric 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 6.0 mA
• D. The saturation current will be 2.0 mA

Answer 1

Answer: (B), (D)

The correct options are
B The stopping potential will be 0.6 V
D The saturation current will be 2.0 mA

The stopping potential or cut-off voltage depends only on the wavelength of the incident wave. It does not depend on the distance between source and photoelectric cell. As same source of light is used in the second case so the stopping potential will be same as previous, $0.6V$.

The saturation current is inversely proportional to square of the distance between source and cell, i.e, $I\propto 1/r^2$

Thus, $\frac{I_2}{I_1}=\frac{r_1^2}{r_2^2}=\frac{(0.2{)}^2}{(0.6{)}^2}=1/9$

So, $I_2=\left(1/9\right)I_1=\left(1/9\right)\left(18\right)=2mA$