In 1905, Einstein published a paper proposing that light was not just a wave, but also could behave as a particle, which he called a "light quantum" (later known as a photon). He used this concept to explain the photoelectric effect, where electrons are emitted from a metal surface when light shines on it.
Einstein's explanation, which was based on Planck's earlier work on blackbody radiation, was a major breakthrough in physics and provided strong evidence for the quantization of light. This work earned him the Nobel Prize in Physics in 1921.
