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A photocell is exposed to a small intense light source kept 1 m from it. If the same source is moved to a distance of 1/2 m, the number of electrons emitted from the photocathode will

1. increase by a factor of 4
2. decrease by a factor of 4
3. increase by a factor of 2
4. decrease by a factor of 2

Correct answer: increase by a factor of 4

Solution

The intensity of light is inversely proportional to the square of the distance from the source. When the light source is moved from 1 m to 0.5 m, the intensity increases by a factor of 4 (since (1/(1/2)² = 4)), leading to a corresponding increase in the number of emitted electrons.

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