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Monochromatic light of wavelength $667 \mathrm{~nm}$ is produced by a helium neon laser. The power emitted is $9 \mathrm{~mW}$. The number of photons arriving per second on the average at a target irradiated by this beam is :
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The correct answer is:
$3 \times 10^{16}$
$$
\begin{array}{r}
\text { No. of photons }=\frac{E}{(\mathrm{hc} / \lambda)} \\
=\frac{9 \times 10^{-3} \times 6.67 \times 10^{-7}}{6.6 \times 10^{-34} \times 3 \times 10^8} \\
=3 \times 10^{16}
\end{array}
$$
\begin{array}{r}
\text { No. of photons }=\frac{E}{(\mathrm{hc} / \lambda)} \\
=\frac{9 \times 10^{-3} \times 6.67 \times 10^{-7}}{6.6 \times 10^{-34} \times 3 \times 10^8} \\
=3 \times 10^{16}
\end{array}
$$
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