In 1900 Max Planck suggested that light (and other forms of electromagnetic radiation) is emitted in indivisible small packets of energy, which he called quanta. (The word ‘photon’ was coined by Wolfers and Lewis in 1926.) The energy, E, of single quantum was found to be proportional to the wave frequency, f:
where the factor h is now known as the Planck constant. In 1905 Albert Einstein used this relationship to describe and interpret how such quanta interacted with surface electrons to explain completely the emission of photoelectrons from a metal surface, a phenomenon known as the photoelectric effect. Einstein was awarded the Nobel prize in 1921 for ‘his discovery of the law of the photoelectric effect’.
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