Spectroscopy studies how matter interacts with electromagnetic radiation. When light hits a substance, it can be absorbed, emitted, or scattered. The pattern of absorption or emission reveals the structure and composition of the substance. Each molecule has a unique spectral fingerprint. Spectroscopy is one of the most powerful tools in chemistry, physics, and astronomy.

The technique comes in many forms. Infrared spectroscopy measures molecular vibrations and identifies functional groups. Ultraviolet-visible spectroscopy measures electronic transitions and is used to quantify concentrations. Nuclear magnetic resonance spectroscopy measures nuclear spin transitions and determines molecular structure. Mass spectrometry is not strictly spectroscopy, but it is often grouped with it. X-ray spectroscopy measures electronic transitions in inner shells. Each form uses a different part of the electromagnetic spectrum.

Spectroscopy is used to identify unknown substances, quantify concentrations, study reaction kinetics, and determine molecular structure. It is used in astronomy to determine the composition of stars and galaxies. It is used in medicine to diagnose disease. It is used in environmental monitoring to detect pollutants. It is used in forensics to analyze evidence.

The instruments vary in complexity. A simple spectrophotometer measures absorbance at one wavelength. An NMR spectrometer uses superconducting magnets and radio waves. A telescope with a spectrograph measures the spectra of distant stars. The common principle is the interaction of light with matter.