Radio wave is the longest kind of electromagnetic wave. Other kinds of electromagnetic waves include gamma rays, visible light, and X rays. A wave can be measured by its wavelength, the distance between successive crests or troughs of the wave. Radio waves have wavelengths greater than 0.04 inches (1 millimeter). A wave can also be measured by its frequency, the number of wave cycles in a given time. Radio waves have frequencies of less than 300 gigahertz, or 300 billion hertz (cycles per second). All electromagnetic waves travel at the speed of light, about 186,000 miles (300,000 kilometers) per second.
Radio waves are used in a variety of communication devices. They are used to broadcast radio and television signals. Radio waves are also used by wireless devices, including cell phones, Global Positioning System (GPS) receivers, satellite communication systems, police radios, and wireless internet devices. Garage door openers and radio-controlled toys also use radio waves. Most radar systems detect objects as varied as airplanes, ships, cars, or clouds by bouncing radio waves off them.
Radio waves are also important in astronomy. In 1932, the American physicist Karl Jansky first detected radio waves from space. Jansky discovered a radio pulse coming from the center of our galaxy. Astronomers have since detected radio waves from planets, supernovae (exploding stars), rapidly spinning pulsars, and galaxies with supermassive black holes at their centers.
A large parabolic (curved) dish serves as the best means of collecting radio waves from space. The American radio engineer Grote Reber built the first radio dish in 1937. One of the world’s largest radio dishes, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Guizhou Province, China, measures about 1,600 feet (500 meters) in diameter. Astronomers also use arrays (groups of dishes), such as the Very Large Array in New Mexico, to increase the sensitivity of their observations.
