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  4. A 200 W sodium street lamp emits yellow light of wavelength 0.6 μ m. Assuming it to be 25 % efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

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Q. A $200\, W$ sodium street lamp emits yellow light of wavelength $0.6 \, \mu m. $ Assuming it to be $25 \% $ efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

AIPMTAIPMT 2012Dual Nature of Radiation and Matter

Solution:

Energy of a photon,
$=\frac{\left(6.6 \times 10^{-34} J s \right)\left(3 \times 10^{8} m s ^{-1}\right)}{0.6 \times 10^{-6} m }$
$=33 \times 10^{20} J$
Number of photons emitted per second is
$N=\frac{\frac{25}{E} P}{E}=\frac{\frac{25}{100} \times 200 W }{33 \times 10^{-20} J }=1.5 \times 10^{20}$