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A 5 watt source emits monochromatic light of wavelength 5000 Å. When placed 0.5 m away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m, the number of photoelectrons liberated will be reduced by a factor of:

1. 8
2. 16
3. 2
4. 4

Correct answer: 4

Solution

The intensity of light decreases with the square of the distance from the source (inverse square law). Doubling the distance (from 0.5 m to 1.0 m) reduces the intensity by a factor of 4, and hence the number of photoelectrons emitted also reduces by a factor of 4.

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