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  • A red laser pointer emits photons of wavelength . If the power of the laser pointer is

A red laser pointer emits photons of wavelength 650 \mathrm{~nm}. If the power of the laser pointer is 5.0 \mathrm{~mW} then the number of photons emitted per second is:

Option: 1

4.51 \times 10^{15}$ photons $/ \mathrm{s}


Option: 2

5.38 \times 10^{15}$ photons $/ \mathrm{s}


Option: 3

1.64 \times 10^{16}$ photons $/ \mathrm{s}


Option: 4

7.67 \times 10^{15}$ photons $/ \mathrm{s}


Answers (1)

best_answer

The energy of each photon can be calculated using the formula E=\frac{h c}{\lambda}, where \mathrm{h} is Planck's constant \left(6.6 \times 10^{-34} \mathrm{Js}\right).
$$ \begin{aligned} E & =\frac{\left(6.6 \times 10^{-34} J \mathrm{~s}\right)\left(3 \times 10^8 \mathrm{~m} / \mathrm{s}\right)}{650 \times 10^{-9} \mathrm{~m}}= \\ 3.05 & \times 10^{-19} J \end{aligned}
The power of the laser pointer is given as P=5.0 \mathrm{~mW}=5.0 \times 10^{-3} \mathrm{~W}.
Using the formula for the number of photons emitted per second, we get:
$$ \begin{aligned} & n=\frac{P}{E}=\frac{5.0 \times 10^{-3}}{3.05 \times 10^{-19}}=1.64 \times \\ & 10^{16} \text { photons } / s \end{aligned}

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shivangi.shekhar

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