Q114 / 125 · BITSAT 2021 · Physics

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:

  • Correct answer
    A
    $3 \times 10^{16}$
  • B
    $9 \times 10^{15}$
  • C
    $3 \times 10^{19}$
  • D
    $9 \times 10^{17}$

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. $\mathrm{N}=\frac{9 \times 10^{-3} \times 667 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^{8}}=3 \times 10^{16} / \mathrm{sec}$

Hence, the correct answer is option: 'A'.

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