1 A range of electromagnetic energies arranged in order of wavelength or frequency (see electromagnetic spectrum). The emission spectrum of a body or substance is the range of radiations it emits when it is heated, is bombarded by electrons or ions, or absorbs photons. The absorption spectrum of a substance consists of dark lines or bands in a continuous spectrum, each line being a wavelength or group of wavelengths at which light is removed from the continuous spectrum by the absorbing medium. These lines and bands are at the same wavelengths as some of the lines and bands in the substance's emission spectrum. Emission and absorption spectra may show a continuous spectrum (also called a continuum), a line spectrum, or a band spectrum. A continuous spectrum contains an unbroken sequence of frequencies over a wide range; continuous spectra are produced by incandescent solids, liquids, and compressed gases. Line spectra are discontinuous lines produced by excited atoms and ions as they fall back to a lower energy level. Band spectra (closely grouped bands of lines) are characteristic of molecular gases or chemical compounds.
2 The coloured band produced when visible light is passed through a spectroscope. See also spectroscopy.