where 
is the frequency of the associated radiation and 
is Planck's constant. [P8.2]
holds: that when electromagnetic radiation is emitted or absorbed by matter, the energy transfers are in terms of the emission or absorption of photons. They are not continuous: arbitrarily small energy transfers are not possible. [P8.2]
holds: that the energy of a photon is given by where is the frequency of the associated radiation and is Planck's constant. [P8.2]
more generally is: a wide ranging and very mathematical branch of quantum theory.
Copyright 1997, The Open University