408 ENERGY MAY BE PROPAGATED BY VIBRATIONS telltale glow that will reveal the outcroppings of scheelite, a tungsten ore. Traces of oil in the mud obtained in oil wells may be detected by the typical fluorescence in ultraviolet rays. Ultraviolet rays are used in making a rapid microscopic test of sputum for the presence of tuberculosis bacteria. The sputum is treated with carbol-auramin, which dyes the bacteria and causes them to glow as bright yellow rods in ultraviolet rays. The use of this technique has greatly decreased the time required for a sputum test. Fig. 180. A portable ultraviolet kit for prospectors. (Courtesy of Ultraviolet Products, Inc.) The lens of the eye is markedly fluorescent, and a light haze in the lens is observed in ultraviolet rays. A special ultraviolet lamp is used to cause the eye to fluoresce and thus helps in cataract surgery. Novel outdoor advertising signs have been made possible by the use of fluorescent paints and lacquers and ultraviolet lamps. The whole sign will appear the same at night as in the daytime when in the light from ordinary incandescent light bulbs; but when these lights are turned off, only those portions of the sign painted with fluorescent paints will glow in the radiations from ultraviolet lamps. During the day the fluorescent materials are invisible. Designers of zeon advertising signs paint miniatures of these signs with fluorescent paints that will glow in ultraviolet rays in such a way as to show exactly how the zeon signs will look at night as well as in the daytime. The availability of powerful and economical sources of ultraviolet radiations would make possible many other interesting light effects; for example, whole ceilings could be constructed of glass panels coated with fluorescent pigments and be caused to glow by ultraviolet sources placed behind them, thus providing a source of diffuse light closely
LIGHT MAY BE PRODUCED BY LUMINESCENCE 409 resembling that of the sky. Panels of translucent plastics containing fluorescent substances could be used in floors, walls, or ceilings and be made luminous by concealed ultraviolet lamps. Houses could be painted with fluorescent paints that would cause them to glow in the radiations from concealed ultraviolet lamps. Fluorescent materials could be dissolved in the water in fountains, exposed to ultraviolet rays. Many inexpensive dyes are fluorescent and thus make possible the manufacture of dyed carpets and dresses, etc. which will fluoresce brilliantly in ultraviolet rays. A number of theatres have their aisles covered with rugs dyed with fluorescent dyes. Special fluorescent lipstick, fingernail polish, face powder, hair preparations, and other cosmetics may be worn for visits to restaurants, theatres, and other places where ultraviolet lamps are used to achieve unusual lighting effects. Fig. 18L "Glowing Carpets" might be the appropriate name to apply to the fluorescent carpet which shines in brilliant colors when irradiated with ultraviolet light. The carpet is used for theater aisles. The ultraviolet lamps used to cause the dyes in the carpet to fluoresce are concealed in the ceilings or elsewhere. (Courtesy of the Calco Chemical Division, American Cyanamid Company.) STUDY QUESTIONS 1. What is the source of the light in nonfilament lamps? 2. What is luminescence? 3. List the different types of luminescence and give an example of each type. 4. Differentiate between phosphorescence and fluorescence. 5. What type of luminescence is illustrated by the firefly? 6. What is the principle of a mercury-vapor lamp? 7. What are the advantages and disadvantages of mercury-vapor lamps? 8. What are the advantages and disadvantages of sodium-vapor lamps? 9. Why are sodium-vapor lamps especially adapted for use on highways? 10. Why do sodium-vapor lamps have a red glow for about a half-hour after they are first turned on? 11. Give a possible explanation of luminescence. 12. Discuss the sources of ultraviolet radiations. 13. Why is it that some ultraviolet lamps are dangerous to use while others are not? 14. Why did the introduction of neon lights revolutionize electrical advertising?