Following on the heels of sonar was another imaging technology that has found an extraordinarily wide range of applications—almost everywhere, it seems, except in medicine. By 1930 researchers at the U.S. Naval Research Laboratory in Washington, D.C., had developed crude equipment that used long waves at the radio end of the spectrum to locate aircraft. Then in 1935, British physicist Robert Watson-Watt designed a more practical radiowave detector that could determine not only range but also altitude. By 1938 dozens of Watson-Watt's devices—called radar, for radio detection and ranging—were linked to form a network of aircraft detectors along Britain's south and east coasts that proved extremely effective against attacking German planes throughout World War II. It is a little-known fact that an hour before the attack on Pearl Harbor, radar detected the incoming planes, though nothing was done with the information. By the end of the war, all the armed powers of the day employed radar in one form or another.

Radar has become one of the most ubiquitous of imaging technologies. Radar detectors create images of weather patterns and support the entire air traffic control system of the United States and other countries. Satellite-borne radar systems have mapped Earth's surface in exquisite detail, independent of weather or cloud cover. Radar aboard spacecraft venturing farther afield have returned images of other planets' surfaces, including stunningly detailed three-dimensional views of Venus obtained right through its otherwise impenetrable blanket of clouds. And, of course, radar is used today in traffic control worldwide, a byproduct of American traffic engineer John Barker's 1947 adaptation of radar to determine an automobile's speed.