When a molecule is irradiated with ultraviolet or visible light, valence shell electrons of the molecule are excited from ground state to higher energy state. For example, in ethylene there are two bond pairs between carbon atoms, one in a-orbital and other in n-orbital. When ethylene absorbs UV radiations, one electron jumps from the bonding molecular orbital to the antibonding molecular orbital.
A molecule will require certain amount of energy for these transitions. This energy is provided by visible and ultraviolet light. Maximum absorption occurs at points where the energy of radiation of certain wave-length becomes equal to the energy required for the excitation. These points of maximum absorption are recorded by a spectrophotometer on a graph, as a function of wavelength. There are two main characteristics of an absorption spectrum; its position and intensity. The position of the absorption band corresponds to the wavelength of radiation whose energy is equal to that required for the electronic transition.
The intensity of absorption depends largely on the probability of molecules in which transition occur and may be measured either as the transmittance (T) or absorbance (A).
Mathematically transmittance is defined as I / I0
Where I0 is the intensity of the incident radiation and I is the intensity of the transmitted radiation. The absorbance also known as optical density .