Population Inversion

population inversion

The condition that occurs when a material is radiated with another material at a certain wavelength, causing electrons to jump to a higher orbit (higher shell). When the electrons return to their ground state, they spontaneously emit a photon. Population inversion is the reason a laser works. The spontaneous emission of photons is amplified by mirrors causing a chain reaction and further stimulation of the excited atoms to release photons. See laser.

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population inversion [‚päp·yə′lā·shən in‚vər·zhən]

(atomic physics)

The condition in which a higher energy state in an atomic system is more heavily populated with electrons than a lower energy state of the same system.

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Population Inversion 

in physics, a state of a substance in which the higher energy levels of the constituent particles (such as atoms or molecules) are more “populated” by particles than are the lower levels. Under ordinary conditions (at thermal equilibrium) the inverse relation exists: there are fewer particles at the higher levels than at lower levels.