The electromagnetic spectrum is typically divided into wavelength or energy regions, based on the characteristics of the waves in each region. Because the properties vary on a continuum, the boundaries are not sharp, but rather loosely defined.
Radio waves are familiar to us due to their use in communications. The standard AM radio band is at 540–1650 kHz, and the FM band is 88-108 MHz. This region also includes shortwave radio transmissions and television broadcasts.
We are most familiar with microwaves because of microwave ovens, which heat food by causing water molecules to rotate at a frequency of 2.45 GHz. In astronomy, emission of radiation at a wavelength of 8.2 in (21 cm) has been used to map neutral hydrogen throughout the galaxy. Radar is also included in this region.
The infrared region of the spectrum lies just beyond the visible wavelengths. It was discovered by William Herschel in 1800 by measuring the dispersing sunlight with a prism, and measuring the temperature increase just beyond the red end of the spectrum.
The visible wavelength range is the range of frequencies with which we are most familiar. These are the wavelengths to which the human eye is sensitive, and which most easily pass through Earth's atmosphere. This region is further broken down into the familiar colors of the rainbow, which fall into the wavelength intervals listed here.
A common way to remember the order of colors is through the name of the fictitious person ROY G. BIV (the I stands for indigo).
The ultraviolet range lies at wavelengths just short of the visible. Although humans do not use UV to see, it has many other important effects on Earth. The ozone layer high in Earth's atmosphere absorbs much of the UV radiation from the sun, but that which reaches the surface can cause suntans and sunburns.
We are most familiar with x rays due to their uses in medicine. X radiation can pass through the body, allowing doctors to examine bones and teeth. Surprisingly, x rays do not penetrate Earth's atmosphere, so astronomers must place x-ray telescopes in space.
Gamma rays are the most energetic of all electromagnetic radiation, and we have little experience with them in everyday life. They are produced by nuclear processes, for example, during radioactive decay or in nuclear reactions in stars or in space.
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High Energy Astrophysics Science Archive Research Center, NASA. "Imagine the Universe. The Electromagnetic Spectrum" [cited February 24, 2003]. <http://imagine. gsfc.nasa.gov/docs/science/know_l1/emspectrum.html>.
K. Lee Lerner