PHOTO ELECTRIC EFFECT

Photoelectric emission -- the ejection of electrons from a metal surface by incident light or other electromagnetic radiation -- is the single richest source of evidence for the particle nature of radiation, and this section develops it from the ground up. It begins with the accidental nineteenth-century observations of Hertz, Hallwachs and Lenard that first hinted something unusual was happening when ultraviolet light struck a metal surface. It then works through a systematic, apparatus-based study of exactly how the resulting photocurrent depends on three independently controllable variables -- the intensity of the light, the potential difference between the electrodes, and the frequency of the light -- distilling the results into five compact experimental laws. Because the classical wave theory of light turns out to be completely unable to explain these laws, the section then introduces Planck's idea that energy comes in discrete packets (quanta) and shows how Einstein extended this into a full quantum explanation of the photoele …