Pictures of the Invisible

No human eye has ever seen a breeze. But two physicists recently produced a photograph of one. They announced a high-speed photography technique by which they have made unusually sensitive pictures of air currents, heat waves, sound waves.

The inventors, Norman F. Barnes and S. Lawrence Bellinger of General Electric Co., showed a picture of the air disturbances at the muzzle of a gun at the moment of firing (see cut). The knots near the muzzle are the hot, expanding gases expelled from the barrel. The long, dark, curved line ahead of them is the "shock wave" of compressed air created when an object travels faster than sound (the smaller curved line at the top of the picture is a shock wave caroming off a metal plate). This phenomenon, which airmen know as "compressibility," has thus far prevented airplanes from flying faster than sound (TIME, July 3).

Scientists have long attempted to make air flow visible by means of a substance such as smoke. But smoke does not follow the intricate air-flow patterns exactly. To get more accurate pictures, the General Electric experimenters and others have developed apparatus based on the simple principle of light refraction.

A beam of light bends when it passes from water to air or vice versa; it also bends when it passes through air masses of varying density. Barnes and Bellinger caught these rapid changes in air density by means of an extremely fast mercury lamp with an exposure of less than one-millionth of a second. The light, flashed through a region of disturbed air, recorded on a photographic plate a "shadowgraph" showing groups of bunched air molecules. Using a more elaborate rig, which has a knife-edge that stops all but the bent light rays, the experimenters developed a technique so sensitive that it can photograph the heat rising from a human hand at room temperature.

This file is automatically generated by a robot program, so reader's discretion is required.