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  4. A Laser light of wavelength 660 nm is used to weld Retina detachment. If a Laser pulse of width 60 ms and power 0.5 kW is used the approximate number of photons in the pulse are: [Take Planck’s constant h = 6.62 × 10-34 Js]

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Q. A Laser light of wavelength $660\, nm$ is used to weld Retina detachment. If a Laser pulse of width $60\, ms$ and power $0.5\, kW$ is used the approximate number of photons in the pulse are :
[Take Planck’s constant $h = 6.62 \times 10^{-34} Js$]

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Solution:

$P=\frac{nhc}{\lambda t} \,\,n=\frac{p\lambda t}{hc}$
$=5\times10^{3}\times \frac{660\times 10^{-9}\times 60\times 10^{-3}}{6.6\times 10^{-34}\times 3\times 10^{8}}$
$=100\times 10^{18}=10^{20}$