The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. Electromagnetic radiation can be divided into octaves — as sound waves are — winding up with eighty-one octaves. The "electromagnetic spectrum" (usually just spectrum) of an object is the frequency range of electromagnetic radiation with wavelengths from thousands of kilometres down to fractions of the size of an atom.
It is commonly said that EM waves beyond these limits are uncommon, although this is not actually 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 infinite.
Spectra of objects [change]
Nearly all objects in the universe emit, reflect or transmit some light. (One hypothetical exception may be dark matter.) The distribution of this light along the electromagnetic spectrum (called the spectrum of the object) is determined by the object's composition. Several types of spectra can be distinguished depending upon the nature of the radiation coming from an object.
NOTE: The AUDIO entry in this graphic is there for comparison only. Sound waves and light are two different things entirely.
- Isaac Asimov, Isaac Asimov's Book of Facts. Hastingshouse/Daytrips Publ., 1992. Page 389.
Other websites [change]
|Wikimedia Commons has media related to: Electromagnetic spectrum|
- U.S. Frequency Allocation Chart - Covering the range 3 kHz to 300 GHz (from Department of Commerce)
- Canadian Table of Frequency Allocations (from Industry Canada)
- UK frequency allocation table (from Ofcom, which inherited the Radiocommunications Agency's duties, pdf format)
- The Science of Spectroscopy - supported by NASA, includes OpenSpectrum, a Wiki-based learning tool for spectroscopy that anyone can edit
- An EM Spectrum Overview in Flash by e-builds