A parallel beam of monochromatic light of wavelength 663 nm is incident on a totally reflecting plane mirror. The angle of incidence is 60° and the number of photons striking the mirror per second is $1.0 ×10^{19}$. The value of the force exerted by it on the surface is:

$10^{-8}N$

Force totally reflecting surface is, $F=\frac{2P\cos 60°}{c}=\frac{2×\frac{nhc}{λ}\cos 60°}{c}=\frac{10^{19}×6.6×10^{-34}}{663×10^{-9}}=10^{-8}N$