Question

A laser produces a beam of light with a wavelength of 532 nm. If the power output is 25.0 mW, how many photons does the laser emit per second? (W = 1 J/s)

Answer 1

The energy of a photon (E) = $\dfrac{hc}{\lambda}$

E = $\dfrac{6.63×10^{-34}×3×10^8}{532× 10^{-9}}$

$= 3.74× 10^{-19}\ J$

Power output = 25mW = $25 × 10^{-3}$W.

This means, the energy emitted in one second = $25 × 10^{-3}$J

Number of photons in one second = $\dfrac{25×10^{-3}}{3.74×10^{-19}} = 6.68 × 10^{16}$