JEE MAIN - Physics (2017 - 9th April Morning Slot - No. 9)
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]
[Take Planck's constant h $$=$$ 6.62 $$ \times $$ 10$$-$$34 Js]
1020
1018
1022
1019
Explanation
The power of the given laser light is expressed as
$$P = {{nhc} \over {\lambda t}}$$
from which the number of photons per second is given by
$$n = P\left( {{{\lambda t} \over {hc}}} \right) = (5 \times {10^2}) \times \left[ {{{(660 \times {{10}^{ - 9}})(60 \times {{10}^{ - 3}})} \over {(6.6 \times {{10}^{ - 34}})(3 \times {{10}^8})}}} \right]$$
$$ = 100 \times {10^{18}} = {10^{20}}$$
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