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 sec on the average at a target irradiated by this beam is:
  1. $3 \times 10^{16}$
  2. $9 \times 10^{15}$
  3. $3 \times 10^{19}$
  4. $9 \times 10^{17}$

Solution

$\lambda=667 \times 10^{-9} m, \mathrm{P}=9 \times 10^{-3} \mathrm{~W}$
$\mathrm{P}=\frac{\text { Nhc }}{\lambda}, \mathrm{N}=$ No. of photons emitted/sec.
$\mathrm{N}=\frac{9 \times 10^{-3} \times 667 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^{8}}=3 \times 10^{16} / \mathrm{sec}$

Asked in: MHT CET Full Test 9

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