The energy change associated with this transition provides information on the structure of the molecule and determines many of its properties, such as colour.
The relationship between the energy involved in the electronic transition and the frequency of radiation is given by Planck's relation.
Sections of molecules which can undergo such detectable electron transitions can be referred to as chromophores, since such transitions absorb electromagnetic radiation (light), which may be hypothetically perceived as color somewhere in the electromagnetic spectrum.
These absorptions are not narrow bands but are generally broad because the electronic transitions are superimposed on the other molecular energy states.
Spectral lines are associated with atomic electronic transitions and polyatomic gases have their own absorption band system.