Radar

Introduced in the 1930s as a system to detect aircraft and surface vessels from far away or in poor weather conditions, radar ("radio detecting and ranging") helped the Allies win World War II. Today, it is one of the greatest contributors to air safety. By bouncing radio waves off moving vessels, ground stations can detect the position of objects in the sky or on the ground. An ingenious spin-off was "stealth technology," which makes objects impervious to radar signals. Modern fighter jets are coated with a material that absorbs radar signals, making the plane "invisible" to radar screens.

Source: http://content.time.com/time/photogallery/0,29307,2026224,00.html