Electromagnetic radiation can interact with matter in a number of ways. If the interaction results in the transfer of energy from a beam of radiant energy to the matter, it is called “absorption”. The reverse process in which a portion of the internal energy of matter converted into radiant energy is called “emission”. In emission process, species in an excited state can emit photons of characteristic energies by returning to lower energy states or ground states.
Part of the radiation which passes into matter, instead of being absorbed, may be scattered or reflected or may be re-emitted at the same wavelength or a different wavelength upon emerging from the sample. Radiation, which is neither absorbed nor scattered, may undergo changes in orientation or polarization as it passes through the sample.