When organic molecules in solution are exposed to light in the region of the spectrum of visible and ultraviolet light, then they absorv light of certain wavelengths depending on the type of electronic transition that is associated with the absorption. The transitions depend on the type of bonding between the electrons within the molecule. The extent of the absorption of light at a particular wavelength can be calculated by using Beer's law. Beer stated that the amount of light absorbed (A) to the concentration of absorbing substance (c) and the length of the path of radiation passing through the sample (b).

A = a x b x c

The a is the constant which stands for the absorptivity of the particular absorbing species. The absoptivity depends not only on the molecule whose absorbance is being determined, but also on the temperature and wavelength of light used in the analysis.

The Guide To Acing Physical Pharmacy