Spectroscopy is the general name given to a very large number of techniques where energy is allowed to fall on a sample and the amount of energy absorbed is measured. Sometimes the light used for analysis has been separated into individual frequencies. It is called monochromatic light, or light of one color. Today we would say that, in monochromatic light, all of the photons have the same energy (or very nearly the same energy).

Spectroscopy often is divided into emission and absorption spectroscopy.