Philipp Lenard and the Photoelectric Effect, 1889-1911

Abstract

Philipp Lenard discovered 1902 that the maximum velocity with which electrons leave a metal plate after it is illuminated with ultra violet light is independent of the intensity of the light. He concluded that in the process of emission the light plays only the role of trigger ing [the release of] a motion which already exists with full velocity inside the atoms of the body.2 He felt that he had found a tool to probe the internal structure of the atom just when atomic theory was coming to the center of physicists' attention. Evidence from the pro duction and absorption of photoelectrons suggested to him 1903 that the atom is dynamic. The atomic volume, he thought, is filled not by a material substance, but by rapidly moving, electrically neutral subatomic units. By 1908, five years before the Bohr atom, Lenard was convinced that discrete spectral lines are emitted whenever an electron returns to a stable configuration the atom. Lenard's early investigation of the photoelectric effect laid the foundation for several influential, if short-lived, ideas about atoms. But Lenard's work on the photoeffect is usually cited a different context today; his study is supposed to have been instrumental the genesis of the light quantum hypothesis. It is frequently claimed that, following Lenard's work, the photoelectric effect constituted one of several difficulties for the wave theory of light.3 The claim would