Question

Monochromatic light of frequaency $6\times {10}^{14}$ 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 sources per second is

• A. $5\times {10}^{14}$
• B. $5\times {10}^{15}$
• C. $5\times {10}^{16}$
• D. $5\times {10}^{17}$

Answer 1

The correct option is B $5\times {10}^{15}$

Energy of incident photon $E=h\nu =6.6\times {10}^{-34}\times 6\times {10}^{14}=3.96\times {10}^{-19}J$
Number of photons per second $N=\frac{P}{E}$
$⟹N=\frac{2\times {10}^{-3}}{3.96\times {10}^{-19}}=5\times {10}^{15}$ photons/second
Correct answer is option B.