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emission spectrum:see spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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emission spectrumA spectrum formed by emission of electromagnetic radiation by matter. Energy has first to be supplied, either as heat (high temperature) or by some other mechanism such as absorption of electromagnetic radiation by the matter or by impact of electrons. The energy raises the atoms or molecules to higher energy levels. In a simple example, the electron in a hydrogen atom may ‘jump’ from its normal orbit to one farther from the nucleus (see hydrogen spectrum). The emission spectrum of hydrogen, or any other emitting material, is formed by transitions from such excited states to lower energy states, the excess energy appearing in the form of photons with characteristic frequencies.
The emission spectrum therefore consists of a specific pattern of narrow peaks – emission lines – that occur at these frequencies. These lines may be superimposed on a continuum, but not necessarily so. Emission lines can occur in stellar spectra if, for example, the star is surrounded by a hot shell of diffuse gas. They also occur, for example, in the spectra of emission nebulae, the interstellar medium (see molecular-line radio astronomy), and quasars. See also absorption spectrum; excitation.