First slide

Photo Electric Effect

Question

When a mono-chromatic point source of light is at a distance r from a photoelectric cell, the cut-off voltage is V and the saturation current is $I$ . If the same source is placed at a distance 3r away from the photoelectric cell, then

Moderate

A

the saturation current will not change
B

the saturation current will change to $\frac{I}{9}$
C

the stopping potential will change
D

the stopping potential will increase to 3V

Solution

When the distance is increased, frequency of incident light do not change and hence, the stopping potential does not change. But the intensity and hence the saturation current decrease nine times.
$I \propto \frac{1}{r^{2}}$

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