The higher energetic beam is passed through a cavity to which it delivers energy. This is done by having a time varying electric field in the cavity. If the frequency of the electric field is tuned such that , resonance condition is satisfied and the molecules make a radiative transition from states with higher energy to that with lower energy.
2.8
Rate Equations for a Two Level System:
Consider a two level system with the upper level and the ground level . In order that laser transition may occur, we need a population inversion. As at normal temperatures, the population of lower level is more than that at the upper level, atoms must be pumped into the upper level by providing them energy equal to the energy difference between the two levels.
in the cavity. If the frequency of the electric field is tuned such that 
, resonance condition is satisfied and the molecules make a radiative transition from states with higher energy to that with lower energy. 
and the ground level 
. In order that laser transition may occur, we need a population inversion. As at normal temperatures, the population of lower level is more than that at the upper level, atoms must be pumped into the upper level by providing them energy equal to the energy difference between the two levels. 
