A parallel beam of light of wavelength 900nm and intensity 100 Wm ^...

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

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

Intensity is defined as power per unit area. Therefore,

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

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

$n = \dfrac{{E}_{net}}{{E}_{singlepholon}} = \dfrac{IA\lambda}{hc}$

$⇒ n = \dfrac{\left(100\right)\left(1 \times {10}^{- 4}\right)\left(900 \times {10}^{- 9}\right)}{6 . 62 \times {10}^{- 34} \times 3 \times {10}^{8}} = 4 . 5 \times {10}^{16}$.