Spectrum
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{{dablink|This article deals with the general meaning of "spectrum" and the history of its use. For other meanings and specific uses, see [[Spectrum (disambiguation)]].}}
[[Image:Rainbow above Kaviskis Lake, Lithuania.jpg|thumb|right|180px|The spectrum in a [[rainbow]]]]
A '''spectrum''' (plural ''spectra'' or ''spectrums''<ref>[http://dictionary.reference.com/browse/spectrum Dictionary.com]. ''The American Heritage Dictionary of the English Language'', Fourth Edition. Houghton Mifflin Company, 2004. (accessed: January 25, 2008).</ref>) is a condition that is not limited to a specific set of values but can vary infinitely within a [[Continuum (mathematics)|continuum]]. The word saw its first scientific use within the field of [[optics]] to describe the [[rainbow]] of colors in visible light when separated using a [[triangular prism (optics)|prism]]; it has since been applied by analogy to many fields. Thus one might talk about the ''spectrum of [[political spectrum|political opinion]]'', or the ''spectrum of activity'' of a drug, or the ''[[autism spectrum]]''. In these uses, values within a spectrum are not necessarily precisely defined numbers as in optics; exact values within the spectrum are not precisely quantifiable. Such use implies a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion.
In most modern usages of ''spectrum'' there is a unifying theme between extremes at either end. Some older usages of the word did not have a unifying theme, but they led to modern ones through a sequence of events set out below. Modern usages in mathematics did evolve from a unifying theme, but this may be difficult to recognize.
==Origins==
In [[Latin]] ''spectrum'' means "image" or "[[apparition]]", including the meaning "[[spectre]]". [[Spectral evidence]] is testimony about what was done by spectres of persons not present physically, or [[hearsay]] evidence about what ghosts or apparitions of Satan said. It was used to convict a number of persons of [[witchcraft]] at [[Salem, Massachusetts]] in the late 17th century.
==Modern meaning in the physical sciences==<!-- This section is linked from [[Edward R. Bradley]] -->
[[Image:K4iii-spectre.png|thumb|right|350px|The spectrum of a [[star]] of [[stellar classification|spectral type]] K4III]]
In the 17th century the word ''spectrum'' was introduced into [[optics]], referring to the range of [[color]]s observed when white light was [[dispersion (optics)|disperse]]d through a [[prism (optics)|prism]]. Soon the term referred to a plot of light [[intensity (physics)|intensity]] or [[power (physics)|power]] as a function of [[frequency]] or [[wavelength]], also known as a [[spectral density]].
The term ''spectrum'' was soon applied to other [[waves]], such as [[sound wave]]s, and now applies to any signal that can be decomposed into frequency components. A spectrum is a usually [[dimension|2-dimensional]] plot, of a compound signal, depicting the components by another measure. Sometimes, the word ''spectrum'' refers to the compound signal itself, such as the "[[Visible spectrum|spectrum of visible light]]", a reference to those [[electromagnetism|electromagnetic]] [[wave]]s which are [[visual perception|visible]] to the [[human]] [[eye]]. Looking at light through a prism separates visible light into its colors according to wavelength. It separates them according to its dispersion relation and a grating separates according to the grating equation and if massive particles are measured often their speed is measured. To get a spectrum, the measured function has to be transformed in their independent variable to frequencies and the dependent variable has to be reduced in regions, where the independent variable is stretched. For this imagine that the spectrum of pulse with a finite number of particles is measured on a film or a CCD. Assuming no particles are lost, any nonlinearity (compared to frequency) on the spectral separation concentrates particles at some points of the film. The same is true for taking a spectrum by scanning a monochromator with a fixed slit width. Violet at one end has the shortest wavelength and red at the other end has the longest wavelength of visible light. The colors in order are violet, blue, green, yellow, orange, red. As the wavelengths get bigger below the red visible light they become infrared, microwave, and radio. As the wavelengths get smaller above violet light, they become ultra-violet, x-ray, and gamma ray.
== See also ==
{{commonscat|Spectrum}}
* [[Electromagnetic spectrum]]
* [[Spectrogram]]
* [[Spectrometer]]
==References==
{{reflist}}
== External links ==
* [http://ioannis.virtualcomposer2000.com/spectroscope/amici.html#colorphotos Emission Spectrum Photos of Various Light Sources]
* [http://physics.nist.gov/PhysRefData/contents-atomic.html NIST Physical Reference Data - Atomic Spectroscopy Data]
[[Category:Science]]
[[Category:Fundamental physics concepts]]
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