A parallel beam of light of wavelength 900   nm and intensity 100 Wm - 2 is incident on a surface…

Wavelength of incident beam $\lambda =900\times {10}^{-9} \mathrm{m}$

Energy of a single photon $E=\frac{hc}{\lambda }$.

Intensity is defined as power per unit area. Therefore,

Intensity of incident beam $I=\frac{E}{tA}$

If the number of photons crossing the given area per second is $n$, then

$n=\frac{E_{\mathrm{net} }}{E_{\mathrm{single} \mathrm{pholon} }}=\frac{IA\lambda }{hc}$

$\Rightarrow n=\frac{\left(100\right)\left(1\times {10}^{-4}\right)\left(900\times {10}^{-9}\right)}{6.62\times {10}^{-34}\times 3\times {10}^8}=4.5\times {10}^{16}$.