Spectrum, rainbowlike series of colors, in the order violet, blue, green, yellow, orange, and red, produced by splitting a composite light, such as white light, into its component colors (see  Color; Light). Indigo was formerly recognized as a distinct spectral color. The rainbow is a natural spectrum, produced by meteorological phenomena. A similar effect can be produced by passing sunlight through a glass prism. The first correct explanation of the phenomenon was advanced in 1666 by the English mathematician and physicist Sir Isaac Newton.

When a ray of light passes from one transparent medium, such as air, into another, such as glass or water, it is bent; upon reemerging into the air, it is bent again. This bending is called refraction; the amount of refraction depends on the wavelength of the light. Violet light, for example, is bent more than red light in passing from air to glass or from glass to air. A mixture of red and violet light is thus dispersed into the two colors when it passes through a wedge-shaped glass prism. See  Optics.

A device for producing and observing a spectrum visually is called a spectroscope; a device for observing and recording a spectrum photographically is called a spectrograph; a device for measuring the brightness of the various portions of spectra is called a spectrophotometer; and the science of using spectroscopes, spectrographs, and spectrophotometers to study spectra is called spectroscopy. For extremely accurate spectroscopic measurements, an interferometer is used. During the 19th century, scientists discovered that beyond the violet end of the spectrum, radiations (see  Radiation) could be detected that were invisible to the human eye but that had marked photochemical action; these radiations were termed ultraviolet (see  Ultraviolet Radiation). Similarly, beyond the red end of the spectrum, infrared radiations were detected that, although invisible, transmitted energy, as shown by their ability to raise the temperature of a thermometer (see  Infrared Radiation). The definition of spectrum was then revised to include these invisible radiations, and has since been extended to include radio waves beyond the infrared, and X rays and gamma rays beyond the ultraviolet (see X Ray) .

The term spectrum is often loosely applied today to any orderly array produced by analysis of a complex phenomenon. A complex sound such as noise, for example, may be analyzed into an audio spectrum of pure tones of various pitches. Similarly, a complex mixture of elements or isotopes of different atomic weights can be separated into an orderly sequence called a mass spectrum in order of their atomic weights .

Spectroscopy has not only provided an important and sensitive method of chemical analysis, but has also been the chief tool for discoveries in the apparently unrelated fields of astrophysics and atomic theory. In general, changes in motions of the outer electrons of atoms produce spectra in the visible, infrared, and ultraviolet regions. Changes in motions of the inner electrons of heavy atoms produce X-ray spectra. Changes in the configurations of the nucleus of an atom produce gamma-ray spectra. Changes in the configurations of molecules produce visible and infrared spectra. See  Electromagnetic Radiation.

Different colors of light are similar in consisting of electromagnetic radiations that travel at a speed of approximately 300,000 km per sec (about 186,000 mi per sec). They differ in having varying frequencies and wavelengths, the frequency being equal to the speed of light divided by wavelength. Two rays of light having the same wavelength also have the same frequency and the same color. The wavelength of light is so small that it is conveniently expressed in nanometers (nm), which are equal to one-billionth of a meter. The wavelength of violet light varies from about 400 to 450 nm, and of red light from about 620 to 760 nm, or from about 0.000016 to 0.000018 in for violet, and from 0.000025 to 0.000030 in for red.