Electromagnetic waves are categorized according to their wavelengths and frequencies, giving the electromagnetic spectrum. These waves are classified as radio, infrared, ultraviolet, etc. Radio waves refer to electromagnetic radiation with wavelengths ranging from millimeters to kilometers. Radio waves are commonly used for audio communications (i.e., radios) and typically result from an alternating current in the wires of a broadcast antenna. They cover a broad wavelength range and are used for AM (amplitude modulated) and FM (frequency modulated) radio, cellular telephones, and TV signals. Microwaves are the highest-frequency electromagnetic waves, produced by currents in macroscopic circuits and devices. Microwave frequencies range from about 109 Hz to nearly 1012 Hz. Most satellite-transmitted information is carried in microwaves. Radar is a common application of microwaves. By detecting and timing the microwave echoes, radar systems can determine the distance to objects as diverse as clouds, aircraft, or even the surface of Venus.
Infrared radiation is generally produced by thermal motion and the vibration and rotation of atoms and molecules. About half of the solar energy arriving to Earth is in the infrared region, with most of the rest in the visible part of the spectrum. Reconnaissance satellites can detect buildings, vehicles, and even individual humans by their infrared emissions.
Visible light is the narrow segment of the electromagnetic spectrum to which the normal human eye responds. We usually refer to visible light as having wavelengths between 400 nm and 750 nm. X-rays are produced by intra-atomic transitions and fast collisions. They are used to image objects that are opaque to visible light, such as the human body or aircraft parts. Gamma-rays are produced during nuclear decay and have the highest frequency in the electromagnetic spectrum.
