392 ENERGY MAY BE PROPAGATED BY VIBRATIONS In these cases the term "ray" refers to straight-line propulsion rather than to wavelike radiations. Infrared Rays Are Heat Rays. Infrared rays, or heat rays, are invisible radiations contained in sunlight which are longer in wave length than the visible light rays. Everyone is familiar with the fact that heat can be radiated. The use of reflectors in electric and gas heating devices shows that heat rays can be reflected just as visible rays are reflected. Smokestacks or airplane exhausts may be detected by the effect of their infrared radiations on sensitive thermocouples six miles away, and icebergs may be located in the fog by their coldness. A "fog-eye" recently developed consists of a reflector which gathers in infrared rays and focuses them on a thermocouple. The electricity generated by the thermocouple is amplified millions of times by amplifiers and may be used to ring warning gongs when a ship is approaching an iceberg. The "fog-eye" can detect a difference of temperature of one fifty-thousandth of a degree, which means that it should respond to the heat of a candle eight miles away. This instrument is mounted on a tripod, which is kept swinging back and forth through a semicircle, scanning the horizon through fog, smoke, or the blackness of night. Had the Titanic been equipped with a "fog-eye," one of the greatest marine disasters of history would have been averted. Thermocouples thus do for infrared radiations what the photoelectric cell does for visible and ultraviolet radiations. A thermoelectric sextant has been devised to measure the position of the sun through obscuring clouds by means of infrared radiations. Inasmuch as infrared rays penetrate the atmosphere better than the radiations of shorter wave length and inasmuch as they affect certain photographic emulsions, infrared rays may be used in photography in cases where the shorter waves are scattered by the atmosphere. Excellent photographs are often made of distant objects on hazy days by use of films especially sensitive to infrared rays and employing an infrared filter that screens out all but the infrared rays. Figure 169 shows what can be done with infrared photography. Pictures have been taken in a dark room with the heat rays from a hot iron, as shown in Figs. 170 and 171. In infrared photographs, the leaves of trees and grass appear white. Because no known green paints have the same appearance in infrared photographs, it is possible to distinguish between natural foliage and camouflage.
LIGHT IS A FORM OF RADIANT ENERGY 393 Moonlight effects are simulated in the movie industry by taking infrared pictures in the daytime. Many stars which are so cool that they do not emit visible light have been discovered by infrared photography. Infrared photography is very useful in examining paintings and in the study of documents and textiles. Fig. 169. The Sierra Nevada range in the vicinity of Yosemite, photographed from Mount Hamilton by infra-red light; distance to Half Dome, 120 miles. (Courtesy of the Lick Observatory.) Infrared lamps are now widely applied in speeding up the baking of enamels and lacquers on automobiles, refrigerators, washing machines, and furniture. By the use of infrared lamps, sheet rubber may be cured in two minutes instead of the two hours formerly required. These lamps may even be used to bake cookies. Soon infrared lamps may be used to dry the family washing or to keep one warm in cold weather. An infrared telescope was patented in 1940. It is based on the fact that fluorescence is speeded up by heat. The telescope focuses infrared rays on a fluorescent screen which is continually flooded with ultra^ violet rays. Where the infrared rays fall upon the screen, the fluorescence ^ increases, and the picture appears. A closed automobile warms up when placed in sunlight because of Fluorescence is the emission of visible light by an object which is irradiated with electromagnetic radiations of shorter wave length.