Description:
In this work we present a summary on the theory of thermal black body radiation in a two-level system using the coefficients A and B of Einstein. Then we study a LASER as a three-level system and we apply Einstein’s theory to determine the Carnot’s thermal efficiency of a LASER. We also use two kinds of kinetic equations for the transitions between two levels and, in equilibrium, we prove that the thermal efficiency is the same as the Carnot efficiency as a function of the temperature of the heat source and the medium. We also use the Carnot cycle in the TS plane to prove that it is the same efficiency assuming that the chemical potential of the radiation is zero.