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  4. How many photons are emitted per second by a 5 mW laser source operating at 632.8 nm ?

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Q. How many photons are emitted per second by a $5\, mW$ laser source operating at $632.8\, nm$ ?

Dual Nature of Radiation and Matter

Solution:

$E=\frac{h c}{\lambda}=\frac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{632.8 \times 10^{-9}} J$
Number of photons per second, $n=\frac{P}{E}$
$\Rightarrow n=\frac{5 \times 10^{-3}}{3.14 \times 10^{-19}}$
$=1.6 \times 10^{16}$