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Question:

Light with an energy flux of 25 × 10⁴ W/m² falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is?

1.25 × 10⁻⁶ N

2.50 × 10⁻⁶ N

1.20 × 10⁻⁶ N

3.0 × 10⁻⁶ N

Solution:

F = 2IA/c = 2(25 × 10⁴)(15 × 10⁻⁴)/3 × 10⁸ = 2.5 × 10⁻⁶ N