Monochromatic light of wavelength $667 \mathrm{~nm}$ is produced by a helium neon laser. The power emitted…
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
$9 \times 10^{17}$
$3 \times 10^{16}$
$9 \times 10^{15}$
$3 \times 10^{19}$
Solution
Here $\lambda=667 \times 10^{-9} \mathrm{~m}, \mathrm{P}=9 \times 10^{-3} \mathrm{~W}$
$\begin{aligned}
\text { Power } & =\frac{\text { energy }}{\text { time }} \\
& =\frac{n h c}{\lambda \mathrm{t}} \\
& =\frac{N h c}{\lambda}
\end{aligned}$
where is number of photons emitted per sec.
$\begin{aligned}
\Rightarrow \quad \mathrm{N} & =\frac{\mathrm{P} \times \lambda}{\mathrm{hc}} \\
& =\frac{9 \times 10^{-3} \times 667 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^8} \\
& =3 \times 10^{16} / \mathrm{s}
\end{aligned}$