O the Theory of Light Quanta.

H. Bateman

H. Bateman

GREGORY A. BREIT

Michael Francis Land

George Birtwistle

Light quanta The history of the corpuscular and wave controversy in optical theory is well known. At the end of the last century physicists had no doubt as to the wave character of light, and the acceptance of the wave theory meant the rejection of the corpuscular theory. The possibility of both being correct and helpful to one another had not been much thought about and still less explored. At about this time Planck's quantum theory appeared and was soon followed in 1905 by Einstein's theory of ‘light quanta’ to explain the photo-electric effect, for which the wave theory alone had utterly failed to account. Einstein assumed that radiation exists in discrete ‘quanta’ of energy hν , where ν is the frequency; thus a quantum is a separate thing, like a corpuscle, and has a frequency associated with it, like a wave. There was a blend of the two older theories involved. This idea has been generalised by the speculations of Louis de Broglie, who pointed out the important clue given by the theory of relativity, namely, that of the identity of mass and energy, so that the conservation of mass is therefore also the conservation of energy. This suggested to him that all forms of energy (including radiation) should have an atomic structure, like matter; and that the atoms of energy are grouped round certain singular points, forming electrons, light quanta, and the like.

Richard C. Tolman, Sinclair Smith

In an earlier number of these Proceedings, (1) Professor G. N. Lewis adopted an extreme form of the light quantum theory, in which the light quanta are assumed to travel in straight lines except when deflected by matter, the facts of interference being explained by the assumption that the quanta do not go to those places where the wave theory forbids the action of radiation. As a crucial test between this and other theories of light, Professor Lewis proposed an experiment in which the absence of light quanta, traveling over a particular path which leads to the center of of an interference band on a screen, would produce an unbalanced torque on a reflecting mirror.