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 $E=hv$
$=6.63\times 10^{-34}\times 6\times 10^{14}J$
Number of photons emitted per second,
$N=P/E$
$=\frac{2\times 10^{-3}}{6.63\times 10^{-34}\times 6\times 10^{14}}$
$=0.05\times 10^{17}=5\times 10^{15}$