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A pulse of light of duration 100 ns is absorbed completely by a small object initially at rest. Power of the pulse is 30 mW and the speed of light is 3 × 10⁸ m/s. The final momentum of object is

1. 0.3 × 10⁻¹⁷ kg m/s
2. 1.0 × 10⁻¹⁷ kg m/s
3. 3.0 × 10⁻¹⁷ kg m/s
4. 9.0 × 10⁻¹⁷ kg m/s

Correct answer: 1.0 × 10⁻¹⁷ kg m/s

Solution

The momentum of light absorbed is given by p = (Power × Time) / c. Substituting the values, p = (30 × 10⁻³ × 100 × 10⁻⁹) / (3 × 10⁸) = 1.0 × 10⁻¹⁷ kg m/s.

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