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Engineering Physics by S. Mani Naidu

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10.5 Population inversion

Usually in a system the number of atoms (N1) present in the ground state (E1) is larger than the number of atoms (N2) present in the higher energy state. The process of making N2 > N1 is called population inversion. Population inversion can be explained with three energy levels E1, E2 and E3 of a system. Let E1, E2 and E3 be ground state, metastable state and excited states of energies of the system respectively such that E1 < E2 < E3. In a system, the population of atoms (N) in an energy level E, at absolute temperature T has been expressed in terms of the population (N1) in the ground state using Boltzmann’s distribution law

N = N1exp(−E / KBT)    where KB = Boltzmann’s constant

Graphically this has been shown in ...

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