Question

When a monochromatic point source of light is at a distance of 0.2 m from a photoelectric 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 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 6.0 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

Since the stopping potential depends on the frequency and not on the intensity and the source is same, the stopping potential remains unaffected. The saturation current depends on the intensity of incident light on the cathode of the photocell which in turn depends on the distance of the source from cathode. The intensity of light is inversely proportional to the square of the distance between the light source and photocell.
Intensity, I $\alpha 1/r^2$ and saturation current $\alpha$ I (Intensity)
$\Rightarrow Saturationcurrent\alpha \frac{1}{r^2}$
$\Rightarrow \frac{(Saturationcurrent{)}_{final}}{Saturationcurrent{)}_{initial}}=\frac{r_{initial}^2}{r_{final}^2}$
$\Rightarrow (Saturationcurrent{)}_{final}=\frac{0.2\times 0.2}{0.6\times 0.6}\times 18=2mA$