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A monochromatic light source of power 5 W emits radiation of wavelength 5000 Å. At a distance of 0.5 m from a photosensitive metal surface, it ejects photoelectrons. If the same source is shifted to 1.0 m from the surface, by what factor will the number of emitted photoelectrons decrease?

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

Correct answer: 4

Solution

Photoelectron count is proportional to the incident intensity, which falls as 1/r^2. Doubling the distance (0.5 m -> 1.0 m) cuts the intensity to 1/4, so the number of emitted photoelectrons decreases by a factor of 4.

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