electromagnetic spectrum
  • The range of energy capable of being transmitted through the universe by the propagation of electrical and magnetic fields, of which the visible spectrum which we experience as light is only a minute fraction. The full electromagnetic spectrum starts with radio waves, which run the gamut from very low frequency to extra high frequency across a range of about 100 000m wavelength to about 1mm. Below this come infrared wavelengths until the longest visible light, the red, is encountered with a wavelength of some 780 nanometres. The visible runs from here to the end of the violet at 380nm, after which the ultraviolet runs up to about 0.01nm. After this come the gamma rays up to about a thousand trillionth of a metre wavelength. X-rays overlap the boundary between ultraviolet and gamma rays. Objects at different temperatures emit radiation preferentially at different wavelengths, getting shorter as the object gets hotter. This forces astronomers wanting to see the whole variety of objects in the universe to look at the whole electromagnetic spectrum. They can now do this with satellites, to allow wavelengths cut out by the Earth’s atmosphere to be observed, and with new tools like radio and infrared telescopes. The radiation reaching the Earth in non-light frequencies is very weak compared to that in visible wavelengths, which is why our eyes have evolved to operate in what we think of as visible light, the band of frequencies in which most radiation from the Sun arrives.
  • The range of frequencies of electromagnetic radiation. This encompasses frequencies from just above 0 Hz to beyond 1024 Hz, corresponding to wavelengths of over 108 meters, to less than 10-16 meters, respectively. These include, in order of ascending frequency: subsonic frequencies, audio frequencies, radio frequencies, infrared light, visible light, ultraviolet light, X-rays, gamma rays, and cosmic rays. These intervals have been arbitrarily established, may be labeled with alternate names associated with specific applications, and may have subdivisions. Also called spectrum (5).

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