438 ENERGY MAY BE PROPAGATED BY VIBRATIONS of window glass result in the light being refracted more at one point than at another. Light passing through homogeneous media having parallel surfaces does not change direction. The angle of refraction in a medium depends upon the angle of incidence, and if the angle of incidence is great enough there will be no Fig. 211. In this type of mirage two images are seen, an apparent one and an actual one. The apparent image is due to the refraction of the light as it enters an upper warmer layer of atmosphere. refraction. This angle, which is the essential condition for total reflection, is known as the critical angle. On a hot day objects on the horizon seem to quiver and are not distinct because the velocity of light in air varies with the temperature; the light is therefore refracted as it passes through layers of air of different temperatures and hence also of different densities. Convection currents of rising hot air become apparent because of the refraction of the light as it passes from cooler to hotter portions of the air. The twinkling of the stars is the result of the refraction of the starlight as it passes through rising hot air currents or falling cold air currents. It has already been pointed out that telescopes are located on mountain tops to avoid this effect as much as possible and that the rooms containing telescopes cannot be heated for the same reason. A type of mirage due to refraction is that produced by the bending of light rays as they pass from chilled surface layers of the air into warmer upper layers, as shown in Fig. 212. A mirage. (From Optics and Wheels, Courtesy of the General Motors Corporation.) Fig. 211. Such a mirage may be formed over water with the result that ships or land below the true horizon become visible. Another type of mirage, illustrated in Fig. 212, is the desert mirage. In this case light from a distant object is bent as it passes from the cooler upper layers of air to the hot surface layers of air until it reaches the critical angle X, at which point the image is reflected as from a mirror. This same effect
LIGHT MAY BE REFLECTED AND REFRACTED 439 is responsible for what appear to be puddles or sheets of water ahead of an automobile on a hot road. The densities of different liquids and solutions can be measured by use of refractometers, which measure the refraction of light in the different media. Such instruments are of great value in the analysis of liquids and solutions. The Lens Is One of the Most Widely Used Optical Devices. The lens is the essential part of telescopes, microscopes, cameras, projectors, and spectacles. rays in the desired direction. All lenses are merely devices to bend light Figure 213 shows how convex and concave lenses bend light rays. Two of the most remarkable applications of lenses are the cystoscope and the gastroscope, which consist of long thin tubes, containing lenses, with a small electric light at one end. This tube is inserted into the bladder or the stomach, as the case may be, to illuminate the interior of the organ. The system of lenses in the tube makes it possible to obtain photographs of the interior of these organs. Fig. 213. The Eye Employs an Adjustable Lens. The eye is similar in principle to the camera, except that it focuses objects by varying the curvature of the lens rather than by varying the distance of the lens from the receiving medium. Nearly 25,000,000 Americans wear glasses, and 60,000,000 should. The majority of errors are errors of refraction, i.e., light rays are not properly focused on the retina. The diagram shows the essential features of the eye, namely: a dark chamber, a lens, and a surface sensitive to light called the retina. The lens of the eye is a double convex type, whose curvature is changed by muscles attached to it so that the eye can focus on near or distant objects. The iris of the eye corresponds to the diaphragm of the camera, similarly dilating or contracting to control the amounts of light passing through the lens.