A parallel beam of intensity 900 W·m⁻² falls on a perfectly reflecting flat mirror of area 0.050 m² at an angle 60° with the mirror normal. Take c = 3×10⁸ m·s⁻¹. If the normal radiation force is approximately N×10⁻⁸ N, find N to the nearest integer.
Physics · Photoelectric effect and dual nature of matter · question based on photoelectric effect
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Question

A parallel beam of intensity 900 W·m⁻² falls on a perfectly reflecting flat mirror of area 0.050 m² at an angle 60° with the mirror normal. Take c = 3×10⁸ m·s⁻¹. If the normal radiation force is approximately N×10⁻⁸ N, find N to the nearest integer.

Physics · Photoelectric effect and dual nature of matter · question based on photoelectric effect

G1 Physics · Accepted Answer

For a perfectly specular reflecting plane at angle θ to the normal, F⊥ = 2IA cos²θ ÷ c. Substitute: I = 900 W·m⁻², A = 0.050 m², cos60° = 0.5. Thus, F⊥ = 2×900×0.050×0.25 ÷ (3×10⁸) = 22.5 ÷ (3×10⁸) = 7.5×10⁻⁸ N. To the nearest integer coefficient, N = 8.