The Photoelectric Effect (1902)

Planck’s explanation of blackbody radiation in terms of quantized energy packets was a major blow to classical thermodynamics and, broadly, to the entire field of physics. A second blow to the classical concept of radiation arose from studies of the photoelectric effect (PE). The term refers to the following phenomenon: Within a chunk of metal, not all the electrons are bound to atoms of the metal; some are free to wander around. (The latter are what make the chunk a conductor, a characteristic feature of metals.) While a free electron is subject to attractive forces from the surrounding atoms, these forces come from all directions and cancel out; the net force on the electron is small. However, if the electron happens to approach the surface of the metal, the forces attracting it back into the interior predominate, so it cannot freely leave the chunk. Each metal has a characteristic value of energy, its work function (Wf), such that an electron just below the surface would need an injection of energy of at least Wf in order to leave the chunk. The work function acts as a barrier to keep the electrons inside the metal until, one way or another, they gain the extra energy. You might think of someone skating on a frozen pond surrounded by a wall. The skater can move freely anywhere on the pond, but can leave the pond only if she gains enough energy to surmount the wall.