IGNOU PHE 9 2026 - An atomic system consisting of two energy levels, with population of h

Question

An atomic system consisting of two energy levels, with population of higher energy level less than that of the lower level, is in thermal equilibrium. Show that the absorption of radiation dominates stimulated emission if radiation of appropriate frequency is introduced into the system. Comment on the consequences of this fact for laser action.

28 Jan 2026

Answer :

Word Count : 377

In an atomic system with two energy levels, let the lower energy level be (E_1) and the higher energy level be (E_2), with populations (N_1) and (N_2) respectively. In thermal equilibrium, according to Boltzmann statistics, the population of the higher energy level is always less than that of the lower energy level: [ \frac{N_2}{N_1} = e^{-(E_2 - E_1)/kT} < 1 ] where (k) is Boltzmann’s constant and (T) is the absolute temperature. This inequality implies

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