Answer
$1.9\times10^{17}$ photons.
Work Step by Step
Energy of one photon
$E=\frac{hc}{\lambda}=\frac{(6.626\times10^{-34}\,Js)(3.00\times10^{8}\,m/s)}{514\times10^{-9}\,m}=3.87\times10^{-19}\,J$
$\text{Total energy}=\text{Power}\times\text{time}=1.5\,W\times0.050\,s=0.075\,J$
$\text{No. of photons}=\frac{\text{Total energy}}{\text{Energy of one photon}}=\frac{0.075\,J}{3.87\times10^{-19}\,J}=1.9\times10^{17}$
