in laser action, the release of energy from an excited atom by artificial means. According to Albert Einstein (Einstein, Albert), when more atoms occupy a higher energy state than a lower one under normal temperature equilibrium (see population inversion), it is possible to force atoms to return to an unexcited state by stimulating them with the same energy as would be emitted naturally.

In stimulated emission the emitted light wave will be coherent (i.e., in phase; see coherence) with the incoming wave. In laser action the stimulating emission triggers a chain reaction in which the radiation from one atom stimulates another in succession until all the excited atoms in the system have returned to normalcy. In doing so, coherent monochromatic light (light of a single wavelength) is emitted.