  Modern attempts to understand light go back to Newton who considered light to
be particles, the so called corpuscular theory, and the other school of
Huygens, Young and others. Huygens and Young viewpoint emphasised the wave
property. This difference of opinions persisted for close to two centuries till
Maxwell theory solidly established light as a wave phenomenon associated with
Electromagnetism. A serious schism so to speak was introduced into the theory
of light with the understanding of "Light gas", the so called Black Body
radiation. Einstein's photon hypothesis and Bose's successful derivation of the
Planck spectrum put the corpuscular theory back in place, while classical
description continues to be used as well. One may then wonder if this being a
classical limit may have to be corrected when dealing with quantum phenomena.
To most people's surprise, the classical description was in fact a subset of
the full quantum description, and the classical states of light could be shown
to be subsumed within the fully quantum description without having to take the
limit of Planck's constant h going to zero. This was the resolution provided by
E. C. G. Sudarshan's Diagonal representation as the most general formalism for
dealing with light.
