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  • A sodium street lamp emits yellow light of wavelength <img alt="0.6 \mu \mathrm{m}" src

A 200 \mathrm{~W} sodium street lamp emits yellow light of wavelength 0.6 \mu \mathrm{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
 

Option: 1

1.5 \times 10^{20}


Option: 2

6 \times 10^{18}


Option: 3

62 \times 10^{20}


Option: 4

3 \times 10^{19}


Answers (1)

best_answer

Give that, only 25 \% of  200 \mathrm{~W} convert  electrical energy into light of yellow colour.

\left(\frac{\mathrm{hc}}{\lambda}\right) \times \mathrm{N}=200 \times \frac{25}{100}

Where \mathrm{N} is the No. of photons emitted per second, \mathrm{h}= plank's constant, \mathrm{c}, speed of light.

\begin{aligned} & \mathrm{N}=\frac{200 \times 25}{100} \times \frac{\lambda}{\mathrm{hc}} \\ & =\frac{200 \times 25 \times 0.6 \times 10^{-6}}{100 \times 6.2 \times 10^{-34} \times 3 \times 10^8}=1.5 \times 10^{20} \end{aligned}
 

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Gunjita

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