Example: Radiation Pressure

Question:
Sunlight with an intensity (= energy flux) S of 1300 W/m² falls perpendicularly on a perfectly reflecting plane mirror of area 0.8 m². How big is the force acting on the mirror?

Answer:

• The force on the mirror can be calculated using Newton's Second Law (basically F=ma): F = Dp / Dt

  where is the Dp momentum transferred to the mirror during the time Dt.

• From our result for radiation pressure, we have Dp = 2 DU / c
• Since the intensity, S, is energy per unit area, A, per unit time, we have S = (1/A) DU / Dt or DU = S A Dt
• We therefore get for the force: F = Dp / Dt = 2 DU / Dt c = 2 S A / c
• Numerical answer: F = 2·(1300 W/m²)·(0.8 m²) / (3.00·10⁸ m/s) = 6.93·10^-6 N

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