Q. Monochromatic light of frequency $6.0 \times 10^{4}$ Hz is produced by a laser. The power emitted is $2 \times 10^{- 3}$ W. The number of photons emitted, on the average, by the source per second is
A) $5 \times 10^{15}$
B) $5 \times 10^{16}$
C) $5 \times 10^{17}$
D) $5 \times 10^{14}$
Explanation
(a) $\ P = 2 \times 10^{- 3}$ W
Energy of photon,
$E = hv$
= 6.6$\times 10^{- 34} \times 6 \times 10^{14}$ J
Where $h$ being Planck’s constant.
Number of photons emitted per second
$n = \frac{p}{\ E}$
= $\frac{2 \times 10^{- 3}}{6.6 \times 10^{- 34} \times 6 \times 10^{14}} = 5 \times 10^{15}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.