  The essential of this monograph is to reproduce the results of Sudarshans
paper Equivalence of semiclassical and quantum mechanical descriptions of
statistical light beams published in 1963. To go in that direction we also
describe the linear harmonic oscillator and its algebraic equivalence with a
single-mode quantized beam of light. Coherent states and their overcompleteness
properties along with the diagonal representation of the density matrix are
essential as a prerequisite to reproduce the result. Most of the textbooks
meant for graduate students describe linear harmonic oscillators in great
detail but they cryptically mention coherent states and their associated
properties. So it is difficult for the students to understand the essence of
Sudarshans paper which contains only seven equations. This monograph is written
in such a lucid manner that without the help of experts one can understand it.
