" This is the key process behind a laser. It’s called stimulated emission, and you can see that it offers a way to increase the number of photons flying along a certain line. Every time a photon hits an excited atom, it duplicates itself, amplifying the amount of light traveling in that direction, which is precisely what the acronym laser stands for: Light Amplification by Stimulated Emission of Radiation. The emission is said to be stimulated (as opposed to spontaneous) because the incoming photon provoked the excited atom into spitting out the new photon. What matters most, however, is that the emitted photon is indistinguishable from the one that spawned it. If you think of these photons not as particles but as tiny waves of light, they’d be perfectly synchronized. "

― Steven H. Strogatz , Sync: The Emerging Science of Spontaneous Order