Normally atoms have the tendency to return to ground state releasing the absorbed energy. Hence, the population of atoms in ground state is greater than that of excited state.
For laser action, there should be more number of atoms in higher energy state.
Population inversion is the state in which the number of atoms in higher energy state is morethan those in lower energy state.
Artificially creating more number of atoms in a higher energy states than the lower energystate is called population inversion. The chances of stimulated emission taking place increases when the state of population inversion is achieved in the medium.
Significance of population inversion in the operation of LASER :
(a)To increase the probability of stimulated emission,the number of atoms in the higher energystate must be greater than the number of atoms in the lower energy state.This is a precondition of LASER.
(b)It makes LASER possible with the help of metastable state.
(c)Amplification of light is ensured because of population inversion.