The measurement and analysis (by means of a spectrometer) of a spectrum is called spectroscopy; it is the study of the interaction between matter and electromagnetic radiation and an essential tool for astronomers, chemists, and physicists. Originally a spectrum was the range of colors observed when white light is dispersed by means of a prism. With the discovery of the wave nature of light, the term spectrum was referred to as the intensity of light as a function of wavelength or frequency. More simply, spectroscopy is the science of measuring the intensity of light at different wavelengths. The graphical representations of these measurements are called spectra.
Most of the different kinds of spectroscopy, corresponding to the various regions of electromagnetic radiation, relate to particular kinds of energy-level transitions. Gamma-ray spectra arise from nuclear energy-level transitions; X-ray spectra from inner-electron transitions in atoms; ultraviolet and visible spectra from outer (bonding) electron transitions in molecules (or atoms); infrared spectra from molecular vibrations; and microwave spectra from molecular rotations.