Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Population inversion occurs when more atoms or molecules in a system are in an excited state than in the ground state. This condition is essential for the operation of lasers.
5 Must Know Facts For Your Next Test
Population inversion is a prerequisite for laser action.
Achieving population inversion requires an external energy source, such as optical pumping or electrical discharge.
In a three-level laser system, population inversion occurs between the second and third energy levels.
In a four-level laser system, the fourth level quickly depopulates to help maintain population inversion.
Population inversion can be described mathematically using the Boltzmann distribution.
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Related terms
Laser: A device that emits light through optical amplification based on the stimulated emission of electromagnetic radiation.
Stimulated Emission: The process by which an incoming photon causes an excited atom or molecule to drop to a lower energy state, releasing a photon identical to the incoming one.
Optical Pumping: A method used to achieve population inversion by using light (photons) to raise electrons from a lower energy level to a higher one.