When a monochromatic point source of light is at a distance 0 . 2 m from a photoelectric cell, the saturation current and cut-off voltage are 12 . 0 mA and 0 . 5 V . If the same source is placed 0 . 4

Question

When a monochromatic point source of light is at a distance 0 . 2 m from a photoelectric cell, the saturation current and cut-off voltage are 12 . 0 mA and 0 . 5 V . If the same source is placed 0 . 4 m away from the photoelectric cell, then the saturation current and the stopping potential respectively are

TestHub · Accepted Answer

The cut-off voltage or stopping potential measure maximum kinetic energy of the electron. It depends on the frequency of incident light whereas the current depends on the number of photons incident. Hence, cut-off voltage will be 0.5 V. Now by inverse square law,

$12 \propto \frac{1}{{(0.2)}^{2}} o r I \propto \frac{1}{{(0.4)}^{2}}$

$∴ \frac{I}{12} = \frac{{(0.2)}^{2}}{{(0.4)}^{2}} = \frac{1}{4}$

or $I = \frac{12}{4} = 3 m A$

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