Spectroscopy deals with the transitions induced in a chemical species by its interaction with the photons of electromagnetic radiation. Spectroscopic methods are generally used to measure the energy difference between various molecular energy levels and to determine the atomic and molecular structures. The instruments used in such studies, called spectrophotometers, are devised to measure the relative energy that is emitted or reflected in the infrared, visible or ultraviolet regions, as a function of wavelength or wave number. Special devices are incorporated into these instruments for the automatic recording of spectra. The spectrum of a molecule is thus presented as a continuous graph obtained by plotting either absorption or transmittance of electromagnetic radiation as a function of wavelength or wave number over a particular range.