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A plate of mass 20 g is held in equilibrium in air by the force exerted by a light beam. Calculate the power of the beam if the plate is perfectly absorbing.

1. 7 x 10⁷ W
2. 6 x 10⁸ W
3. 6 x 10⁷ W
4. 7 x 10⁸ W

Correct answer: 6 x 10⁸ W

Solution

For a perfectly absorbing surface, the force exerted by light = Power/c (where c = 3 x 10⁸ m/s, speed of light). For equilibrium: Force = weight. mg = 0.020 * 10 = 0.2 N. Power = Force * c = 0.2 * 3 x 10⁸ = 6 x 10⁷ W.

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