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11.Dual Nature of Radiation and matter
medium
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 photon of yellow light it emits per second is
A
$1.5 \times 10^{20}$
B
$62 \times 10^{20}$
C
$3 \times 10^{19}$
D
$6 \times 10^{18}$
(AIPMT-2012)
Solution
Effective power $=\frac{25}{100} \times 200 W=50 W$
Now, $50=n h v=\frac{n h c}{\lambda}$
$n=\frac{50 \lambda}{h c}$
$n=\frac{50 \times 0.6 \times 10^{-6}}{6.6 \times 10^{-34} \times 3 \times 10^8}=1.5 \times 10^{20}$
Standard 12
Physics
