\(=\frac{6.63 \times 10^{-34}}{500 \times 10^{-9}}=1.33 \times 10^{-27} \mathrm{\,kg}-\mathrm{m} / \mathrm{s}\)

and no. of photons

\(=\frac{0.5}{\mathrm{hv}} / \mathrm{cm}^{2}=\frac{0.5 \lambda}{\mathrm{hc}} / \mathrm{cm}^{2}\)

\(=\frac{0.5 \times 500}{1240 \times 1.6 \times 10^{-19}} / \mathrm{cm}^{2}\)

\(=1.25 \times 10^{18}\) \(\mathrm{photons}\) \(/ \mathrm{cm}^{2}\)

\(\therefore\) force \(=1.25 \times 10^{8} \times 1.33 \times 10^{-27} \times \pi \times 1^{2}\)

\(=5.22 \times 10^{-9} \mathrm{\,N}\)