While others before him, like Thomas Young, demonstrated the wave-like nature of light through experiments like the double-slit experiment, Einstein took it a step further by explaining the photoelectric effect in 1905.
His work showed that light could also behave as a particle (what we now call a photon), explaining how light can knock electrons off of metal surfaces.
Therefore, Einstein's work brought forth the concept of light's wave-particle duality, a fundamental concept in quantum physics.
