A perfectly reflecting mirror has an area of 1 cm². Light energy is allowed to fall on it for 1 h at the rate of 10 W cm^-2. The force that acts on the mirror is:

$6.7 \times 10^{-8} N$

Let, E = energy falling on the surface per second = 10 J

Momentum of photons, $p=\frac{h}{\lambda}=\frac{h}{(c / v)}=\frac{h v}{c}=\frac{E}{c}$

On reflection, change in momentum per second = force

= 2 p = $\frac{2 E}{c}=\frac{2 \times 10}{3 \times 10^8}=6.7 \times 10^{-8} N$