/spek"treuhm/, n., pl. spectra /-treuh/, spectrums.1. Physics.a. an array of entities, as light waves or particles, ordered in accordance with the magnitudes of a common physical property, as wavelength or mass: often the band of colors produced when sunlight is passed through a prism, comprising red, orange, yellow, green, blue, indigo, and violet.b. this band or series of colors together with extensions at the ends that are not visible to the eye, but that can be studied by means of photography, heat effects, etc., and that are produced by the dispersion of radiant energy other than ordinary light rays. Cf. band spectrum, electromagnetic spectrum, mass spectrum.2. a broad range of varied but related ideas or objects, the individual features of which tend to overlap so as to form a continuous series or sequence: the spectrum of political beliefs.[1605-15; < L: appearance, form, equiv. to spec(ere) to look, regard + -trum instrumental n. suffix]
* * *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.
* * *▪ physicsin optics, the arrangement according to wavelength of visible, ultraviolet, and infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an instrument that photographs or maps spectra is a spectrograph. Spectra may be classified according to the nature of their origin, i.e., emission or absorption. An emission spectrum consists of all the radiations emitted by atoms or molecules, whereas in an absorption spectrum, portions of a continuous spectrum (light containing all wavelengths) are missing because they have been absorbed by the medium through which the light has passed; the missing wavelengths appear as dark lines or gaps.The spectrum of incandescent solids is said to be continuous because all wavelengths are present. The spectrum of incandescent gases, on the other hand, is called a line spectrum because only a few wavelengths are emitted. These wavelengths appear to be a series of parallel lines because a slit is used as the light-imaging device. Line spectra are characteristic of the elements that emit the radiation. Line spectra are also called atomic spectra because the lines represent wavelengths radiated from atoms when electrons change from one energy level to another. Band spectra is the name given to groups of lines so closely spaced that each group appears to be a band, e.g., nitrogen spectrum. Band spectra, or molecular spectra, are produced by molecules radiating their rotational or vibrational energies, or both simultaneously.
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