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Arrangement according to wavelength (or frequency) of electromagnetic radiation. The visible, rainbow spectrum is the portion of the electromagnetic spectrum that is visible as light to the human eye. Some sources emit only certain wavelengths and produce an emission spectrum of bright lines with dark spaces between. Such line spectra are characteristic of the elements that emit the radiation. A band spectrum consists of groups of wavelengths so close together that the lines appear to form a continuous band. Atoms and molecules absorb certain wavelengths and so remove them from a complete spectrum; the resulting absorption spectrum contains dark lines or bands at these wavelengths.
This entry comes from Encyclopædia Britannica Concise. For the full entry on spectrum, visit Britannica.com.