Monochromatic light of frequency $6.0 \times 10^{14} \;Hz$ is produced by a laser. The power emitted is $2.0 \times 10^{-3} \;W$.

$(a)$ What is the energy of a photon in the light beam?

$(b)$ How many photons per second, on an average, are emitted by the source?

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$(a)$ Each photon has an energy $E=h v=\left(6.63 \times 10^{-34} J s \right)\left(6.0 \times 10^{14} Hz \right)$

$=3.98 \times 10^{-19} \,J$

$(b)$ 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 $P=N E .$ Then

$N=\frac{P}{E}=\frac{2.0 \times 10^{-3} \,W }{3.98 \times 10^{-19}\, J }$

$=5.0 \times 10^{15}$ photons per second.

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