Visible light is one form of electromagnetic radiation. In empty space, electromagnetic radiation travels at 186,282 miles (or 299,792 kilometers) per second. The frequency, or wavelength, of electromagnetic radiation - the distance between oscillations - is usually measured in ångströms (Å). One ångström is equal to a hundred-millionth of a centimeter. At one end of the visible spectrum, what we usually refer to when we speak of light, is high-frequency, short-wavelength violet light. Violet light has a wavelength of about 4000 Å. At the other end of the visible spectrum is low-frequency, long-wavelength red light. Red light has a wavelength of about 7200 Å. Below the red end of the visible spectrum lies infrared radiation, which has a longer wavelength than red light; and below infrared radiation lie microwaves and then radio waves, which can have wavelengths many miles long. Beyond the violet end of the visible spectrum lie types of electromagnetic radiation with ever shorter wavelengths: ultraviolet light, X-rays, and gamma rays. The entire sequence of types of electromagnetic radiation, arranged according to wavelength, is known as the electromagnetic spectrum, a representation of which is shown below.
The longer wavelength forms of electromagnetic radiation - infrared, microwaves, and radio waves—are commonly used to transmit information through the air. Different portions of the available electromagnetic spectrum are set aside for different forms of communication - radio, television, cellular telephony, mobile marine communications, and so on. Two radio or television stations in close proximity to one another cannot broadcast at the same frequency because their signals would interfere with one another. Therefore, it is necessary to regulate what frequencies are used by whom and where. In the United States an agency of the federal government, the Federal Communications Commission, or FCC, has responsibility for regulating, in the public interest, who uses which frequencies. The FCC also auctions off available parts of the electromagnetic spectrum.