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Radiation and Radioactivity : The Radiation Spectrum
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Light is a mixture of colors. White light can be dispersed into its component colors: violet, indigo, blue, green, yellow, orange, and red. This ordered arrangement of colors is called the visible spectrum. A particular color of light can be specified by either its frequency or its wavelength with the property that the frequency multiplied by the wavelength is equal to the speed of light, about 300 million meters per second (about 186,000 miles per second).
The rainbow is a natural spectrum, produced by meteorological phenomena. A similar effect can be produced by sunlight passing through a glass prism. The English mathematician and physicist Sir Isaac Newton advanced the first correct explanation of the phenomenon in 1666. 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.
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 the above devices to study spectra is called spectroscopy.
For accurate spectroscopic measurements, an interferometer is used. During the 19th century, scientists discovered that beyond the violet end of the spectrum, radiation could be detected that was invisible to the human eye but that had marked photochemical action; this radiation was termed ultraviolet. Similarly, beyond the red end of the spectrum, infrared radiation was detected that, although invisible, transmitted energy, as shown by their ability to raise the temperature of a thermometer. The definition of spectrum was then revised to include this invisible radiation, and has since been extended to include radio waves beyond the infrared, and X-rays and gamma rays beyond the ultraviolet.
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