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What is the radiation pressure produced by an electromagnetic wave of intensity I (W m⁻²) on a perfectly absorbing, nonreflecting surface? [c is the speed of light]

1. Ic
2. Ic²
3. I/c
4. I/c²

Correct answer: I/c

Solution

For a perfectly absorbing surface the radiation pressure equals the intensity divided by the speed of light, P = I/c. (For a perfect reflector it would be 2I/c.)

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