Q. How many photons are emitted by a laser source of $5 \times 10^{- 3}\ W$ operating at 632.2 nm in 2s?
A) $3.2 \times 10^{16}$
B) $1.6 \times 10^{16}$
C) $4 \times 10^{16}$
D) None of these
Explanation
(a) Energy of each photon
$E_{0} = \frac{hc}{\lambda} = \frac{6.63 \times 10^{- 34} \times 3 \times 10^{8}}{632.2 \times 10^{- 9}} = 3.15 \times 10^{- 19}\ J$
Total energy $= E = Pt$
$\therefore nE_{0} = Pt$
$\Rightarrow n = \frac{Pt}{E_{0}} = \frac{5 \times 10^{- 3} \times 2}{3.15 \times 10\hat{} - 19} = 3.2 \times 10^{16}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.