S. Bose, PLANCK LAW AND THE LIGHT QUANTUM HYPOTHESIS (REPRINTED FROM AMERICAN JOURNAL OF PHYSICS, VOL 44, 1976), Journal of astrophysics and astronomy, 15(1), 1994, pp. 3
The phase space of a light quantum in a given volume is subdivided int
o ''cells'' of magnitude h3. The number of possible distributions of t
he light quanta of a macroscopically defined radiation over these cell
s gives the entropy and with it all thermodynamic properties of the ra
diation.