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A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed 1/2 m away, the number of electrons emitted by photocathode would

Asked in AIEEE 2005 · Intensity, area and distance

Answer: (4) increase by a factor of 4

Step-by-step solution

A point source spreads its power over a sphere, so the intensity at the cell follows I∝1/(r²).

Halving r multiplies the intensity by 4.

The number of electrons per second follows the number of photons per second, hence the intensity.

So the count increases by a factor of 4.

Why the other options are wrong

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