In 1924, Satyendra Nath Bose (then at Dacca University) re-derived Planck's law by developing a new kind of statistics obeyed by light quanta. He asked Albert Einstein to consider the resulting manuscript for publication in Zeitschrift für Physik. Einstein translated it into German and published it with the comment:1
In my opinion Bose's derivation of Planck formula signifies an important advance.
Apparently, while translating, Einstein made changes in the manuscript submitted by Bose; Narasimhaiengar Mukunda wrote:2
In his paper sent to Einstein, Bose apparently made another radical suggestion - that each photon has an intrinsic angular momentum or helicity of exactly one (quantum) unit, which could be either parallel or antiparallel to its momentum direction. But—revolutionary as he was—Einstein found this suggestion too revolutionary and removed it in the published version of Bose's paper!
Bose had also mentioned this suggestion to Partha Ghose.3 But the most important document is the 1931 Raman–Bhagvantam paper4 that reported the experimental determination of photon's spin. They wrote
In his well-known derivation of the Planck radiation formula from quantum statistics, Prof. S N Bose obtained an expression for the number of cells in phase space occupied by the radiation, and found himself obliged to multiply it by a numerical factor 2 in order to derive from it the correct number of possible arrangements of the quantum in unit volume. The paper as published did not contain a detailed discussion of the necessity for the introduction of this factor, but we understand from a personal communication by Prof. Bose that he envisaged the possibility of the quantum possessing besides energy hv and linear momentum hv/c also an intrinsic spin or angular momentum round an axis parallel to the direction of its motion. The weight factor 2 thus arises from the possibility of the spin of the quantum being either right-handed or left-handed, corresponding to the two alternative signs of the angular momentum.5
We may mention here that the intrinsic angular momentum of the electron was not postulated until 1925 by Uhlenbeck and Goudsmit.6
Bose had used “light quantum” in his paper, which in 1926 came to be known as photon. Richard Beth's experiment, which also established the angular momentum of the photon, came much later.7
One obvious question is how Bose could deduce the angular momentum of the photon to be . In 1909, Poynting was probably the first scientist to derive the expression for the angular momentum carried by a circularly polarized electromagnetic wave; he suggested that a wave with energy density u would contain angular momentum density .8 Bose must have incorporated the Einstein equation into Poynting's expression to discover that the angular momentum of the light quantum was or
Bose was correct in postulating that there were only two possible angular momentum states, rather than also allowing zero angular momentum. This fact was confirmed in 1939 by Eugene Wigner9 who used quantum field theory to show that zero rest mass relativistic particles of any spin have only two helicity states.
