Question

In the above question how many photons per second on an average are emitted by the source?

• A. $4\times 10^{15}$ photons per second
• B. $5\times 10^{15}$ photons per second
• C. $6\times 10^{15}$ photons per second
• D. $8\times 10^{15}$ photons per second

Answer 1

Answer: (B)

If $N$ is the number of photons emitted by the
source per second, the power $P$ transmitted in the beam
equals $N$ times the energy per photon $E$, so that $P=NE$
$\Rightarrow N=\frac{P}{E}=\frac{2\times 10^{-3}W}{3.98\times 10^{-19}J}$
( $E$ was calculated in the last question)
$\Rightarrow N=5\times 10^{15}{s}^{-1}$