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A laser diode emits light of wavelength \lambda=780 \mathrm{~nm} with a power of P = 20 mW. How many photons are emitted per second from the laser diode?

Option: 1

1.94 \times 10^{15} \mathrm{~s}^{-1}


Option: 2

2.45 \times 10^{15} s^{-1}


Option: 3

3.67 \times 10^{15} s^{-1}


Option: 4

3.67 \times 10^{15} s^{-1}


Answers (1)

best_answer

The energy of each photon can be calculated using the formula

E=\frac{h c}{\lambda} , where h is Planck's constant and c is the speed of light.

Substituting the values, we get:

\begin{aligned} E & =\frac{\left(6.626 \times 10^{-34} \mathrm{~J} \cdot \mathrm{s}\right)\left(3 \times 10^8 \mathrm{~m} / \mathrm{s}\right)}{780 \times 10^{-9} \mathrm{~m}}= 8.06 \times 10^{-20} \mathrm{~J} \end{aligned}

The number of photons emitted per second can be calculated using the formula n=\frac{P}{E} .

Substituting the values, we get:

n=\frac{20 \times 10^{-3} \mathrm{~W}}{8.06 \times 10^{-20} \mathrm{~J}}=2.48 \times 10^{15} \mathrm{~s}^{-1}

Rounding off to the correct number of significant figures, we get the answer as (2)

2.45 \times 10^{15} s^{-1}

Therefore, option (2) is the correct answer.

 

Posted by

sudhir.kumar

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