The famous physicist who proposed a theory explaining the photoelectric effect was Albert Einstein. In 1905, he introduced the concept that light can be both a wave and a particle, describing light particles as photons. His explanation of the photoelectric effect demonstrated that when light strikes a material, it can eject electrons if the light's frequency exceeds a certain threshold. This groundbreaking work solidified the foundation for quantum mechanics and enhanced the understanding of the dual nature of light, marking a significant milestone in the field of physics.