What is Electromagnetic spectrum?
The '''electromagnetic''' '''spectrum''' is the range of all possible [[electromagnetic radiation]]. Electromagnetic radiation can be divided into [[octave]]s — as [[sound]] waves are — adding up to eighty-one octaves.<ref>Isaac Asimov, ''Isaac Asimov's Book of Facts''. Hastingshouse/Daytrips Publ., 1992. Page 389.</ref> [[Physicist]]s have studied electromagnetic radiation with [[wavelength]]s from thousands of [[kilometre]]s down to fractions of the size of an [[atom]]. It is commonly said that waves beyond these limits are uncommon, although this is not known to be true. The short wavelength limit is likely to be the [[Planck length]], and the long-wavelength limit is the size of the [[universe]] itself, though in principle the [[spectrum]] is [[infinity|infinite]]. Radiation of shorter wavelength than about 30 [[Micrometre|μm]] is commonly detected by its ability to exceed the [[ionization energy]] of atoms. Radiation of longer than 3[[Millimetre|mm]] is commonly detected by its ability to induce [[electrical current]]s. Radiation between these limits was little used until the 21st century because of the difficulty of detecting it. [[Image:TheElectromagneticSpectrum.jpg|thumb|center|500px|Electromagnetic spectrum]] == Spectra of objects == Nearly all objects in the [[universe]] emit, reflect or transmit some light. ([[Black hole]]s do not.) The distribution of this light along the electromagnetic spectrum (called the ''spectrum'' of the object) is determined by what the object is made of. Several types of spectra can be distinguished depending upon the nature of the radiation coming from an object. [[Spectroscopy]] is the branch of [[physics]] that observes matter by its emitted or reflected spectra. == References == {{reflist}} == Other websites == {{commonscat|Electromagnetic spectrum}} * [http://www.ntia.doc.gov/osmhome/allochrt.html U.S. Frequency Allocation Chart] - Covering the range 3 kHz to 300 GHz (from Department of Commerce) * [http://strategis.ic.gc.ca/epic/internet/insmt-gst.nsf/vwapj/spe