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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 emitted 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 emitted per second is

NTA AbhyasNTA Abhyas 2022

Solution:

Efficient power $p=\frac{N}{t}\times \frac{h c}{\lambda }=200\times 0.25$
$\frac{N}{t}=50\times \frac{\lambda }{h c}$
$=\frac{50 \times 0.6 \times 10^{- 6}}{6.6 \times 10^{- 34} \times 3 \times 10^{8}}$
$=1.5\times 10^{20}$