laser

A laser is a device that produces an intense, narrow, single-colour beam of light in which all the waves march in step. The name is an acronym — light amplification by stimulated emission of radiation — and that mouthful is really the whole story: light is amplified because each passing photon coaxes an excited atom into emitting a fresh photon that is an exact copy of the first. Ordinary lamps and the Sun pour out light in every colour and direction at once; a laser pours out light that is almost perfectly orderly.

Three ingredients make it work. A gain medium — a gas, a crystal, or a semiconductor — holds atoms that can be pumped into excited states; an energy source keeps pumping them up faster than they fall back down, so that more atoms are excited than relaxed; and a pair of mirrors forms a cavity that sends the growing wave back and forth through the medium, building it up like a child pumping a swing in rhythm. One mirror leaks slightly, and through that leak the laser beam escapes.

Because the photons are coherent — locked in the same frequency, direction, and phase — a laser beam stays tight over long distances, can be focused to a tiny bright spot, and carries a single pure wavelength. That orderliness is what makes lasers useful for everything from reading a barcode and cutting steel to carrying internet traffic through glass fibres and performing delicate eye surgery. A laser is one of the most direct everyday payoffs of quantum mechanics.