The German physicist Albert Einstein (1879 to 1955), famous for his theories of relativity, conducted research on the photoelectric effect, in which electrons fly out of a metal surface exposed to light.

The strange thing about the photoelectric effect is the energy of the electrons (photoelectrons) that fly out of the metal does not change whether the light is weak or strong. (If light were a wave, strong light should cause photoelectrons to fly out with great power.)

Another puzzling matter is how photoelectrons multiply when strong light is applied. Einstein explained the photoelectric effect by saying that "light itself is a particle," and for this he received the Nobel Prize in Physics.