Planck's Quantum Theory of radiation:
According to this theory,radiant energy is emitted or absorbed not continuously but discontinuously in the form of quanta.Each quanta is associated with definite amount of energy.
The energy (E)associated with each quantum foot is a particular radiation of frequency 'v' is given by E=hv where h is planck's constant.
In the case of light,the quantum of energy is called photon.
A body can emit or absorb either one quantum of energy or some whole number multiple of it i.e,hv,2hv,3hv etc called as quantisation of energy.