Photon Emission Rate Of A Laser

The number of photons emitted per second is the total radiated power divided by the energy of one photon, since each photon carries an indivisible quantum of energy. The single-photon energy is $E=\frac{hc}{\lambda }=\frac{6.6\times 10^{-34}\times 3\times 10^8}{660\times 10^{-9}}=3.0\times 10^{-19}\text{J}$, equivalent to about $1.88\text{eV}$. The emission rate is then $N=\frac{P}{E}=\frac{2\times 10^{-3}}{3.0\times 10^{-19}}\approx 6.6\times 10^{15}$ photons per second. The value $3.3\times 10^{15}$ wrongly doubles the photon energy. The value $1.3\times 10^{16}$ halves it. The value $2.0\times 10^{18}$ ignores the photon energy and divides power by the electronic charge instead. This calculation makes vivid how enormously many photons constitute even a feeble milliwatt beam, which is why the granular, quantised nature of light is hidden in everyday optics, a point underscored in the NCERT dual-nature chapter. A plausibility check confirms the answer is a very large but finite number, consistent with light appearing perfectly continuous to our senses despite being made of discrete quanta. It is instructive to appreciate that this staggering rate of roughly seven thousand million million photons each second is what makes the beam appear perfectly smooth and continuous to the eye and to ordinary detectors. Only with extremely sensitive single-photon counters operated at very low light levels does the discrete, grainy arrival of individual quanta become directly observable, confirming the particle character of light.