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

10^{20}

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10^{18}

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10^{22}

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10^{19}

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Solution

The correct option is C $10^{20}$
Power due to laser $= \frac{Δ n}{Δ t} . \frac{h c}{λ}$
$Δ n \to$ Number of photons per pulse
$Δ t = 60 m s$
$0.5 \times 10^{3} W = \frac{Δ n}{60 \times 10^{- 3}} \times \frac{6.62 \times 10^{- 34} \times 3 \times 10^{8}}{660 \times 10^{- 9}}$
$996.97 \times 10^{17} = Δ n$
$Δ n = 0.99 \times 10^{20}$
$Δ n \approx 10^{20}$

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