The intensity of a plane electromagnetic wave is $I$. The radiation pressure $P$ exerted by this wave on a perfectly reflecting surface at normal incidence is:

Solution

1. The momentum of a photon is $p=\frac{E}{c}$.
2. For a perfectly reflecting surface, the change in momentum per photon is $\Delta p=2\frac{E}{c}$.
3. Pressure is defined as force per unit area, $P=\frac{1}{A}\frac{dp}{dt}$.
4. Since $I=\frac{1}{A}\frac{dE}{dt}$, the pressure is $P=\frac{2I}{c}$.
5. Hence the answer is (B).