434 EN ERGY MAY BE PROPAGATED BY VIBRATIONS The success of signal buoys at sea and the red and white reflectors now used so widely on bicycles, signs, guard rails, and house numbers depends upon this principle. There are two taillights on modern automobiles, which enable one to judge how far ahead a car is at night. If the lights appear to be close together the car is far ahead, but if they spread apart rapidly you are approaching the car ahead at a fast rate of speed. Modern taillights (reflex have sections reflectors) which will reflect Fig. 202. Beam from a reflex reflector ^.^^ lig^^ from a car behind if the type of taillight. taillight bulb burns out. The sky would appear black if it were not for the fact that the molecules and dust of the atmosphere diff'use the light of the sun and moon by reflecting a portion of the light. It can be shown that it is the dust particles that are responsible for this difi^usion of the light of the sun which causes the changing colors at sunrise and sunset, because, when light is passed through a dust-free box, it is invisible although it can be seen before it enters and after it leaves the box; light cannot be seen unless it is coming toward one. Thus a powerful beam of light from a searchlight could be passed through a dust-free room and leave it complete darkness. The room would be illuminated at once, however, by placing a white object in the path of the light. A useful application of this idea is the placing of a white or light-colored object in front of an automobile to reflect the light from the headlights when one is caught with engine or tire trouble without a flashlight on a dark night. The images of objects in flat mirrors differ from the original only in that they are reversed. Mirror-writing is a common example of this effect. Invisible Glass Has Many Practical Uses. C. Hawley Cartwright discovered that a film of magnesium fluoride placed on glass makes it invisible by rendering it nearly perfectly transparent. Ordinary glass reflects about 8 per cent of the incident light and transmits about 92 per cent, while this new coated glass transmits 99.6 per cent of the incident light. For the films to be rugged enough to be practical in most cases, the transmission is increased to only 98 per cent. Reducing the reflection 5 per cent for each surface, however, reduces glare and ghosts 25 per cent. This invention promises to have widespread application in eliminating the glare from showcases, store windows, and glass-covered paintings. in
LIGHT MAY BE REFLECTED AND REFRACTED 435 It has already been applied in making camera lenses "faster." This "speeding-up" of lenses is especially valuable in color photography and television. The thickness of the film is four millionths of an inch, and it must be carefully controlled because the secret of the success of the film depends upon its thickness. A film of this thickness is such that the light rays reflected by the two surfaces of the film cancel each other by interference. Concave and Convex Mirrors Are Used Like Lenses to Focus or Diverge Light. Mirrors of curved shape are used as reflectors in automobile headlights. Light from a distant object will be brought to a focus at the point where the lamp is located. This is the principle of the reflecting telescope, flashlights, and parabolic reflectors of automobile headlights. On the other hand, an automobile lamp placed at this point of focus will reflect long Fig. 203. This reflection on the face of C. Hawley Cartwright shows how a thin film decreases the light reflection from glass. Sunlight is reflected from the glass to his cheek. The round spot in the center of the piece of glass was coated with a thin film of magnesium fluoride. (Courtesy of Science Service.) parallel beams. Concave mirrors can produce three kinds of images depending upon the position of the object: (1) if the object is very close to the mirror, the image will be upright but larger than the object — dentists' exploratory mirrors use this principle; (2) if the object is placed somewhat farther from the mirror, the image will be smaller than the object and Fig. 204. In the automobile headlight the lamp is placed at the focal point of the concave mirror.