Which physicist explained the photoelectric effect using light quanta in 1905?

Albert Einstein explained the photoelectric effect using light quanta in 1905.

The photoelectric effect occurs when light ejects electrons from a material. Einstein proposed that light arrives in discrete packets, later called photons, whose energy is proportional to frequency. An electron absorbs one photon, so higher-frequency light gives each photon more energy, while brighter light supplies more photons.

This explanation accounted for observations that classical wave theory could not easily explain. Electrons are emitted only above a threshold frequency, emission begins without a measurable delay, and increasing intensity mainly increases the number of emitted electrons rather than their maximum energy.

Einstein received the 1921 Nobel Prize in Physics, awarded in 1922, specifically for his discovery of the law of the photoelectric effect. The effect is often confused with Compton scattering, which concerns changes in the wavelength of X-rays or gamma rays after collisions with electrons.