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Light with an energy flux of \( 25 \times 10^4 \mathrm{Wm}^{-2} \) falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is:

1. 1.25 × 10⁻⁶ N
2. 2.50 × 10⁻⁶ N
3. 1.20 × 10⁻⁶ N
4. 3.0 × 10⁻⁶ N

Correct answer: 2.50 × 10⁻⁶ N

Solution

The force exerted by light on a perfectly reflecting surface is given by the formula: F = 2(P/c), where P is the power incident on the surface and c is the speed of light. First, calculate the power: P = energy flux × area = (25 × 10⁴) × (15 × 10⁻⁴) = 375 W. Then, F = 2 × (375 / 3 × 10⁸) = 2.50 × 10⁻⁶ N.

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