Einstein’s proposal of light quanta

Einstein’s 1905 “photoelectric paper”, where he first proposed the idea of light quanta, was concerned mainly with blackbody radiation. Using the equipartition theorem of statistical mechanics, he was the first one to correctly derive the Rayleigh–Jeans law. Classical physics can explain the low-frequency region of Planck’s distribution; Einstein then concentrated on the high-frequency part, the Wien limit law, as representing the new physics. He investigated radiation entropy’s volume-dependence, showing that it had an analogous expression to that for the Joule expansion of an ideal gas. This allowed him to suggest that in the high-frequency limit the blackbody radiation could be interpreted as a gas of quanta (later called photons). Einstein then proposed to test this idea by the properties of the photoelectric effect. Upon the experimental verification of his predictions by Millikan, he was awarded the 1921 Nobel Prize in physics.