Question

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

• A. ${10}^{20}$
• B. ${10}^{18}$
• C. ${10}^{22}$
• D. ${10}^{19}$

Answer 1

Answer: (A)

${10}^{20}$

Power due to laser $=\frac{\Delta n}{\Delta t}.\frac{hc}{\lambda }$

$\Delta n\to$ Number of photons per pulse

$\Delta t=60ms$

$0.5\times {10}^{3}W=\frac{\Delta n}{60\times {10}^{-3}}\times \frac{6.62\times {10}^{-34}\times 3\times {10}^8}{660\times {10}^{-9}}$

$996.97\times {10}^{17}=\Delta n$

$\Delta n=0.99\times {10}^{20}$

$\Delta n\approx {10}^{20}$