The number of red photons $(\lambda =663 \mathrm{nm})$ that must strike a totally reflecting screen per second at normal incidence so that a force of $1 \mathrm{N}$ is exerted on the screen is

Because photons rebound with the initial value of momentum

$F=\frac{\Delta p}{\Delta t}=\left(\frac{N}{\Delta t}\right)\left[\frac{h}{\lambda }-\left(-\frac{h}{\lambda }\right)\right]=n\left(\frac{2h}{\lambda }\right)$

So, total number of photons striking the totally reflecting screen per second $\left(n=\frac{N}{∆t}\right)$ is

$n=\frac{F}{\left(\frac{2h}{\lambda }\right)}=\frac{F\lambda }{2h}=\frac{1\times 663\times {10}^{-9}}{2\times 6.63\times {10}^{-34}}=5\times {10}^{26}$