Q149 / 200 · NEET 2012 (Screening) · Physics
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
- Correct answerA$1.5 \times 10^{20}$
- B$6 \times 10^{18}$
- C$62 \times 10^{20}$
- D$3 \times 10^{19}$
Solution
Efficient power $P=\frac{N}{t} \times \frac{h c}{\lambda}=200 \times 0.25$
$\begin{aligned}
& \frac{N}{t} =50 \times \frac{\lambda}{h c} \\
& =\frac{50 \times 0.6 \times 10^{-6}}{6.6 \times 10^{-34} \times 3 \times 10^8} \\
& =1.5 \times 10^{20}
\end{aligned}$
Hence, the correct answer is option: 'A'.