Question

A laser produces a beam of light of frequency $5 \times 10^{14}$ Hz with an output power of 33 mW. The average number of photons emitted by the laser per second is (Planck's constant = $6.6 \times 10^{-34}$ Js)

• A. $40 \times 10^{16}$
• B. $10 \times 10^{16}$
• C. $30 \times 10^{16}$
• D. $20 \times 10^{16}$

Hint:

Energy of photon = $h\nu$. Number of photons/sec = Power/($h\nu$).

Answer 1

The Correct Option is B

The energy of a single photon is given by $E = h\nu$, where $h$ is Planck's constant and $\nu$ is the frequency. $$ E = (6.6 \times 10^{-34} \, \text{Js})(5 \times 10^{14} \, \text{Hz}) = 33 \times 10^{-20} \, \text{J} $$ Power is the energy emitted per unit time. The output power is 33 mW, which is $33 \times 10^{-3} \, \text{J/s}$. The number of photons emitted per second (n) is given by: $$ n = \frac{\text{Power}}{\text{Energy per photon}} = \frac{33 \times 10^{-3}}{33 \times 10^{-20}} = 10^{17} = 10 \times 10^{16} $$