An instrument for investigating quantitatively the way in which electromagnetic radiation of different wavelengths is absorbed by a specimen. Typically, it consists of a source of the radiation (infrared, visible, or ultraviolet) from which a particular wavelength can be selected by means of a monochromater. A beam of this radiation is passed through the specimen (or reflected from it) and the intensity measured by a detector (e.g. a photocell). As the wavelength of incident radiation is changed, an *absorption spectrum
is produced showing how the intensity of transmitted (or reflected) radiation varies with wavelength. Typically, substances absorb the radiation over particular bands of wavelength, corresponding to energy-level changes in their molecules. Spectrophotometry is used extensively as a means of analysis and in the study of how radiation interacts with matter (e.g. the absorption of light in photosynthesis). The speed of a chemical reaction can also be followed in cases where the products of the reaction absorb radiation of a particular wavelength that the reactants do not.