Discussion
A wave of power density 100 W/m² strikes a perfect reflector normally. The radiation pressure is about
Answer: D
Correct answer: 0.67 \( \mu \)Pa
A perfect reflector reverses the wave’s momentum rather than absorbing it, so the pressure is twice that on a black surface: \[ p = \frac{2S}{c} = \frac{2\times100}{3\times10^8} = 6.7\times10^{-7}\ \mathrm{Pa}. \] The 0.33 microPa offered omits the factor of two and is the absorbing case. The magnitudes explain why solar sails must be enormous - even full sunlight gives only a few micropascals.
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