Question

Monochromatic light of frequency $6.0 \times 10^{ 14} \,Hz$ is produced by a laser. The power emitted is $2 \times 10^{ - 3} \,W$. The number of photons emitted, on the average, by the source per second is

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

Answer 1

Answer: (D)

Power of monochromatic light beam is $P= Nhv$
where $N$ is the number of photons emitted per second.
Power $P=2 \times 10^{-3} W$
Energy of one photon $E=h v$
$=6.63 \times 10^{-34} \times 6 \times 10^{14} J$
Number of photons emitted per second,
$ N=P / E $
$= \frac{2 \times 10^{-3}}{6.63 \times 10^{-34} \times 6 \times 10^{14}}$
$=0.05 \times 10^{17}=5 \times 10^{15}$