A beam of total power 60 W falls normally on a plane surface. The surface absorbs 30% of the incident power, specularly reflects 50%, and transmits the remaining 20% without changing the propagation direction. Take c = 3×10⁸ m·s⁻¹. If the radiation force on the surface is N×10⁻⁸ N, find N.
Physics · Photoelectric effect and dual nature of matter · question based on photoelectric effect
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Question

A beam of total power 60 W falls normally on a plane surface. The surface absorbs 30% of the incident power, specularly reflects 50%, and transmits the remaining 20% without changing the propagation direction. Take c = 3×10⁸ m·s⁻¹. If the radiation force on the surface is N×10⁻⁸ N, find N.

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

G1 Physics · Accepted Answer

For normal incidence, absorbed radiation transfers momentum P(abs) ÷ c per second, while specularly reflected radiation transfers twice its incident momentum. Therefore, F = [P ÷ c][A₁+2R₁], where A₁ is absorptivity and R₁ is reflectivity. Here, A₁ = 0.30, R₁ = 0.50. Thus, A₁+2R₁ = 0.30+1.00 = 1.30. Therefore, F = 60×1.30 ÷ (3×10⁸) = 78 ÷ (3×10⁸) = 2.6×10⁻⁷ N = 26×10⁻⁸ N. Hence N = 26.