Einstein's photon hypothesis

In 1905, his 'miracle year', Einstein proposed that light itself — not just the oscillators that emit it — is made of discrete energy packets, each carrying E = hf. Planck had quantized only the act of emission and absorption and hoped light remained a continuous wave; Einstein took the far bolder step of granting the quantum a real, independent existence as a particle of light. He called them light quanta; today we call them photons.

With this single assumption the baffling features of the photoelectric effect dissolved into common sense. One photon strikes one electron and surrenders its whole energy hf at once. Part of that energy frees the electron from the metal; the rest becomes its kinetic energy. This predicts immediately why there is a threshold frequency, why electron energy grows with frequency but not brightness, and why brighter light just means more ejected electrons.

Einstein's prediction, K_max = hf − W, was a clean straight line waiting to be tested. Robert Millikan, who disliked the idea and set out to disprove it, instead confirmed it beautifully around 1915. It was specifically for this work, not for relativity, that Einstein received the 1921 Nobel Prize — a reminder that the quantum revolution had as many reluctant midwives as eager ones.