Absolutely Small, Chapter 4: The Photoelectric Effect and Einstein's Explanation

In 1905, Einstein explained the photoelectric effect. Although best know for his Theory of Relativity, Einstein actually won a Nobel Prize for his explanation of the photoelectric effect because it was fundamental to the development of quantum theory. When light shines on certain metals, electrons come flying out. The way in which this happens could not be explain with the classical mechanics theory of light. Classical theory said light is a wave. EinsteinAÆs explanation of the photoelectric effect showed that light acts like particles he called photons.