One thorn in the argument for light-as-a-wave purists is a phenomenon called the photoelectric effect. When light shines on a metal surface, electrons fly out. But higher intensity of light does not cause more electrons to be released, as you would expect with the wave theory. Albert Einstein studied this effect and came up with a compelling theory that stated light was both wave and particle. Light flows toward a metal surface as a wave of particles, and electrons release from the metal as an interaction with a single photon, or particle of light, rather than the wave as a whole. The energy from that photon transfers to a single electron, knocking it free from the metal. Einstein’s declaration of wave-particle duality earned him the Nobel Prize in physics in 1921.