Who first explained the photoelectric effect using quantum theory?

Albert Einstein first explained the photoelectric effect using quantum theory. In 1905, he proposed that light transfers energy in discrete packets, later called photons, rather than only as a continuous electromagnetic wave.

The photoelectric effect occurs when light strikes a material and ejects electrons. Experiments showed that the electrons’ maximum energy depends on light frequency, not simply on brightness. Light below a material’s threshold frequency produces no emission, even if its intensity is increased; brighter light above the threshold generally produces more emitted electrons.

Einstein expressed the energy balance as E = hν, with each light quantum carrying energy proportional to frequency. Some of that energy overcomes the material’s work function, and the remainder becomes the electron’s kinetic energy. Max Planck had introduced energy quanta in black-body radiation, but Einstein applied the quantum idea directly to light and the photoelectric effect.

Robert Millikan later confirmed Einstein’s equation experimentally. Einstein received the 1921 Nobel Prize in Physics for his work on the photoelectric effect, not specifically for relativity.