onsider a system of atoms with two energy states E1 and E2 with N1 and N2 number of atoms per unit volume in each energy states respectively. The N1 and N2 are called the number densities of the atoms. Let a radiation of energy density Eν of frequency ν be incident on the system. In the case of induced absorption, an atom in the ground state E1 goes to an excited state E2 by absorbing a suitable photon of energy hν = E2 − E1. The number of such absorptions per unit time, per unit volume is called the rate of induced absorption. This depends on the number density N1 of the ground state and the energy density of the incident radiation Eν. That is,