Basic physical principle of laser action was introduced in Chapter 1. We interpreted the laser action as a result of amplification by multiplying the number of photons inside the cavity containing the medium (atoms) and feedback of the energy to build up sustained oscillation. In this chapter we attempt a semi classical model in terms of a self consistent theory. An electric field present within the medium produces microscopic dipole moments; these moments added together in a many-atom system lead to a macroscopic polarization. This polarization acts as a source in the Maxwell’s equations producing a reaction field. We demand that the reaction field is the same as the field that induced the process. This is the condition of self-consistency. The process can be described by a flow chart (Fig. 5.1) for the semi classical model. The electric field produces a dipole moment that can be explained by using the quantum mechanical model of the interaction of the classical electromagnetic field with an atom having quantized energy levels. Principles of statistical mechanics are used by considering the medium as an ensemble and density matrix is used to facilitate the process of calculating the macroscopic polarization. Self-consistent model where the generating field E(r,t) is equal to the reaction field E’(r,t). https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492396/2543d099-b354-4900-b875-fd425f903a05/content/fig5_1.tif"/>