Quanta
Before 1900, the term "quanta" (singular "quantum") was used to describe particles or amounts of various quantities, including electricity. The significant shift in its usage came in 1900 when the German physicist Max Planck was studying black-body radiation. Planck suggested that experimental observations, especially at shorter wavelengths, could be explained if the energy within a molecule was a "discrete quantity composed of an integral number of finite equal parts," which he termed "energy elements."
In 1905, Albert Einstein built upon Planck's idea while studying light-related phenomena such as black-body radiation and the photoelectric effect. Einstein proposed that these phenomena could be better explained by modelling electromagnetic waves as consisting of spatially localized, discrete wave-packets. He called these wave-packets "light quanta."
Photon
The term "photon" derives from the Greek word for light. It was initially suggested as a unit related to the illumination of the eye and the resulting sensation of light. This term was used in a physiological context by several scientists:
1916: American physicist and psychologist Leonard T. Troland.
1921: Irish physicist John Joly.
1924: French physiologist René Wurmser.
1926: French physicist Frithiof Wolfers.
Although Wolfers's and Lewis's theories were contradicted by many experiments and not widely accepted, the term "photon" gained popularity. Arthur Compton used "photon" in 1928, referring to Gilbert N. Lewis, who coined the term in a letter to Nature on 18 December 1926. Despite earlier uses of the term, it was Lewis's coinage that became widely adopted among physicists.
