A parallel beam of monochromatic light of wavelength 550 nm falls normally on a plane metallic surface which is perfectly reflecting. The power of the beam is 5 mW and its cross-sectional area is 1.0 mm². Find the force and pressure exerted by the radiation on the metallic surface:

$3.3×10^{-11}N,3.3×10^{-5}N/m^2$

As, $I=\frac{P}{A}=\frac{5×10^{-3}}{1×10^{-6}}=5×10^3W/m^2$

Force on the surface, $F=\frac{2P}{c}=\frac{2×5×10^{-3}}{3×10^8}=3.3×10^{-11}N$

Pressure on the surface, $=\frac{2I}{c}=\frac{2×5×10^{-3}}{3×10^8}=3.3×10^{-5}N/m^2$