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 :
  1. $3 \times 10^{19}$
  2. $9 \times 10^{17}$
  3. $3 \times 10^{16}$
  4. $9 \times 10^{15}$

Solution

$\begin{array}{r} \text { No. of photons }=\frac{E}{(\mathrm{hc} / \lambda)} \\ =\frac{9 \times 10^{-3} \times 6.67 \times 10^{-7}}{6.6 \times 10^{-34} \times 3 \times 10^8} \\ =3 \times 10^{16} \end{array}$

Asked in: NEET 2009 (Mains)

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