In 1839, French physicist, Edmund Bequerel noticed that shining light on certain metals could produce a tiny electrical current. In 1905 Albert Einstein expanded upon and tied together the work of Bequerel, Max Planck and other physicists to provide a more complete explanation of the phenomenon, that light has the characteristics of both waves and particles. The electrons released when a light shines on a metal are called photoelectrons. The energy of a photon in a beam of light is proportional to the frequency of that beam of light. Metals have a property called the threshold frequency, below which no photoelectrons will be released. Light must exceed the threshold frequency in order to provide the electrons within the outermost shell of a metal atom enough energy to leave their bounded state. Another way of understanding photon absorption is to look at the band gap. The band gap defines the minimum amount of energy required for an electron to escape its bound state.