Electromagnetic radiation in the visible and ultraviolet regions, wavelengths of 200 nm to 800 nm, of the electromagnetic spectrum can be absorbed by molecules that contain multiple bonds. The UV or Visible light energy is absorbed when the electrons are excited from their ground state to some higher energy level. A UV/Vis spectrophotometer is designed to measure the amount of light absorbed at each wavelength of the ultraviolet and visible light regions.
The absorbance of the light energy can then be plotted against the wavelength of light to give an absorbance spectra. The wavelengths at which maxima occur are characteristic of the absorbing species and the height of the peaks give us information about the concentration of the absorbing species.
The instrument pictured is a Shimadzu UV-Vis 2401PC Spectrophotometer (purchased in 2006).