A laser light of wavelength 660nm is used to weld Retina...

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

Power$\left(P\right)$ is given as, $P = \dfrac{E}{t} = \dfrac{nhc}{\lambdat}$

From the above equation, the number of photons is $n = \dfrac{P\lambdat}{hc}$

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

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