1. Quantization of Radiation (Planck's Hypothesis) .
"If a physical system executes a simple harmonic motion in one dimension with frequency v, it take only those energy values E which are given by the relation".
2. Blackbody Radiation .
"Any object which is a good absorber of radiation of a particular wavelength is also a good emitter of radiation of the same wavelength. A body which absorbs radiation of all wavelengths is called a blackbody".