quantum theory /kwon″təm/ [L, quantum, how much; Gk, theoria, speculation] , (in physics) the theory dealing with the interaction of matter and electromagnetic radiation, particularly at the atomic and subatomic levels, according to which radiation consists of small units of energy called quanta. Radiation can be absorbed only in whole quanta, and the energy content of a quantum is inversely proportional to its wavelength. Energy and frequency are expressed as E = hω. E is the energy of the quantum; h is the Planck constant; and ω is the frequency of the radiation. Also called quantum mechanics. See also Planck’s constant.
