Basis of quantum theory, that the energy of electromagnetic waves is confined in indivisible packets or quanta, each of which has to be radiated or absorbed as a whole, the magnitude being proportional to frequency. If E is the value of the quantum expressed in energy units and v is the frequency of the radiation, then E=hv, where h is known as Planck's constant and has dimensions of energy x time, ie action, and is 6-626 x 10~34 J s. See photon and panel on Quantum theory.

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