400 ENERGY MAY BE PROPAGATED BY VIBRATIONS yellow-green light. The great disadvantage of these lamps is that the faces of people take on a deathlike appearance in their yellow-green light, and colors cannot be matched. This difficulty is partly overcome by the use of reflectors, on which red fluorescent materials have been placed so as to add the red rays that are missing, and by locating incandescent lamps in the same fixture. A new type of mercury-vapor lamp used in television studios because it gives such an intense, yet cool, light is the water-cooled mercuryvapor quartz-tube light. The lamp itself is a quartz tube of very fine bore about 13^ inches long, about the size of a cigarette. Tungsten wires are sealed into each end ; and a small amount of mercury, together with argon at a pressure of about one pound per square inch, is contained in the tube. When the electricity is turned on, the mercury vaporizes and produces a pressure of about 1200 pounds per square inch. The tube produces a great deal of heat and must be cooled by water. Its light output is 65,000 lumens, and it is K as bright as the sun. Mercury-vapor lamps are of special interest because they are excellent sources of ultraviolet radiations, which cause fluorescence. Sodium-vapor Lamps Produce a Yellow Light That Is Certain T3rpes of Illumination. Excellent for The sodium-vapor lamp is another type of lamp which employs electroluminescence. The sodium-vapor lamp employs the principle of the mercury-vapor lamps, except that sodium replaces the mercury. The sodium-vapor lamp contains neon gas for starting and thus has the characteristic red color of neon luminescence until the lamp gets warm enough to vaporize the sodium. Sodium-vapor lights are especially adapted for safety lighting for highways, bridges, and underpasses, their chief advantage being their relatively high efficiency as compared with tungsten-filament lamps. Electromagnetic Waves Shorter than Those of Visible Light Produce Fluorescence and Phosphorescence. Some materials, when irradiated by light of one wave length (color), will emit light of a different, but always longer, wave length. Such radiations are called "luminescence," or "cold light." The explanation is based on the electronic theory of the atomic structure, to be discussed in the next Unit. It is sufficient to state here that according to the electron theory this light is emitted when electrons which have been displaced from their normal or stable position in the atoms by bombardment with some form of energy particles return to their original condition.
LIGHT MAY BE PRODUCED BY LUMINESCENCE 401 Cathode rays, X rays, gamma rays, and, in general, any type of electromagnetic radiation having wave lengths shorter than those of visible light can produce luminescence. Ultraviolet rays are widely applied to produce luminescence. If the luminescence lasts during the period of excitation only, it is called "fluorescence." If the luminosity persists after removal of the exciting radiation, it is called "phosphorescence." Fig. 174. Sodium vapor lamps illuminate the San Francisco-Oakland Bay Bridge. (Courtesy of the Pacific Gas and Electric Company.) Zinc sulfide may be obtained in different forms, which show a red, yellow, blue, or green phosphorescence. Phosphorescent paints are used for theatrical displays, exits, danger signals, signs, and advertising purposes. Sources of Ultraviolet Light. Small argon lamps, which can be operated in ordinary light sockets and by ordinary lighting current, give off a light containing ultraviolet rays. When ultraviolet light alone is desired, filters of special glass which absorb the visible light rays are employed.