The conception of photons

In the year 1900 Max Planck was led by experimental observations to propose a strange formula for the intensity as a function of frequency for light emitted by a cavity. It relied on peculiar properties to be obeyed by the emitters and absorbers in the cavity. I highlight the mathematically suggestive nature of the formula, accessible even to a high school student, that could have provided a clue to the physical reasoning of Planck. In 1905, Einstein made the bold photon hypothesis and was able to predict the formula for the photoelectric effect which was not fully explored then. The first part of this article concerns the period 1905 to 1923 with a possible explanation for the difficulties in acceptance of the new concept. In the second part of the article I present how S. N. Bose's 1924 paper provided a systematic derivation of Planck formula by consistently adopting the conception of photons within Boltzmann's framework. In conclusion we discuss the implications of Quantum Mechanics that eventually emerged, showing that the seeds of some of its uncanny conceptual content were already foreshadowed in Einstein's proposal.