436 ENERGY MAY BE PROFAC.ATEI) BY VIBRATIONS inverted; (3) if tiie object is placed still farther away, the image will still Fig. 205. An all-glass driving lamp, similar in construction to a sealed-beam headlight except that it employs one filament instead of two filaments. (Courtesy of the General Electric Company.) be inverted, but it will be larger than the object. The bowl of a spoon or the sidcrcar-vicw mirror of an automobile are convex mirrors, in which the image is always smaller than the object. Sealed-beam reflectors were introduced on automobiles in 1940. These headlights seal the interior from dust and moisture and thus preserve the reflecting surface. One type of sealed-beam headlight uses a silver-plated brass reflector, a lamp, a glass lens, and a gasket, while a second type uses a glass lens fused to a glass reflector filled with argon gas in which the lamp filaments are located without the use of a separate bulb. The use of offset filaments in an automobile headlight controls the beam in the two-beam headlight system. See Fig. 206. By placing one filament slightly to the side of the other, the beam that tilts down also turns somewhat to the right. See Fig. 207. The Refraction of Light Explains Many Phenomena. The velocity of light is different in media of different densities. A crucial test of the two theories of light was based on this fact. Accord- Behind Ahead Fig. 206.
LIGHT MAY BE REFLECTED AND REFRACTED 437 ing to Newton's corpuscular theory, light would travel through water with a greater velocity than it would travel through air, and the reverse would be true of light if propagated by waves. In 1850 the French physicist, Jean Leon Foucault, reestablished Huygens' wave theory by showing that light travels through water with a velocity about three Fig. 208. marching in the sand will be slowed down, with a consequent change in the Just as the men who are direction of line of march, so light rays are bent as they pass through the prism. quarters of that of light through air. When light passes from one medium to another, it is bent (refracted) because of the fact that it travels with a different velocity in the second medium. Thus a pencil placed in a glass of water appears Fig. 209. A pencil appears bent at the point where it dips into the water. to be bent at the surface of the water. If a coin is placed at the bottom of an evaporating dish slightly below the line of vision, it will be invisible to the audience until water is poured into the dish. See Fig. 210. Water appears to be shallower than it really is because of refraction. We actually see the sun about 83^ minutes after it has gone below the horizon because the light from the sun is bent toward the earth by the atmosphere. Plate glass does not lend to objects the distorted appearance frequently seen with common window glass because it is of uniform thickness and density, whereas /////^^////////////////////W////^//77Z thevariationsin the thicknessand density Fig. 210.