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A beam of radiation carrying energy E is incident perpendicularly on a perfectly reflecting surface. What momentum is imparted to the surface? (C = speed of light)

1. 2E/C
2. 2E/C²
3. E/C²
4. E/C

Correct answer: 2E/C

Solution

Radiation of energy E carries momentum E/c. On a perfectly reflecting surface the photons reverse, so the impulse imparted is 2E/c.

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