A laser light of wavelength 660   nm is used to weld Retina detachment. If a laser pulse of width 60…

The energy of photon is given by $E=\frac{hc}{\lambda }$.

Power$\left(P\right)$ is given as, $\mathrm{P}=\frac{E}{t}=\frac{nhc}{\lambda t}$

From the above equation, the number of photons is $n=\frac{P\lambda t}{hc}$

$=0.5\times {10}^3\times \frac{660\times {10}^{-9}\times 60\times {10}^{-3}}{6.6\times {10}^{-34}\times 3\times {10}^8}$

Hence, number of photons in the pulse are$=100\times {10}^{18}={10}^{20}$.