440 ENERGY MAY BE PROPAGATED BY VIBRATIONS Near-sightedness and far-sightedness are the results of elongated and shortened eyeballs respectively. People who have these defects either cannot bring near or distant objects into focus, or they subject their eyes to undue strain in so doing. Such defects are corrected by the use of concave or convex lenses. Another common eye trouble is astigmatism. Astigmatism will cause more discomfort and eyestrain than any other type of eye defect. The eyeballs in this case are slightly cylindrical in shape, so that two Fig. 214. The diflferent types of eye defects and the types of lenses required to correct these defects. equally distant lines at right angles to each other will not be in the same focus. Astigmatism is corrected by use of a cylindrical lens. Double vision or unequal focus, which at its worst is represented by crossed eyes, is due to muscle unbalance. Extreme cases may be overcome by delicate operations on the muscles, and less severe cases can be corrected by means of prismatic lenses. Aniseikonia, Greek for "unequal images," is a condition in which the images received by the two eyes differ in size and shape. People afflicted with this defect lack depth perception and distance sense and should not operate automobiles or airplanes. The eye strain resulting from this disorder frequently causes headaches and stomach or nervous disorders. Aniseikonia can be corrected by "tailor-made" lenses. STUDY QUESTIONS 1. Mention three types of eye defects. What is the cause in each case? How is each type corrected? 2. W'hat is the principle involved in red reflectors? 3. What is meant by reflection and refraction of light? 4. Why does the sky appear light on the earth's surface in the day, while it is very dark at higher altitudes? 5. Why does a pencil placed in a glass of water appear to be bent at the surface of the water?
LIGHT MAY BE REFLECTED AND REFRACTED 441 6. What causes the stars to twinkle? 7. Explain the desert mirage. 8. Explain the hot-pavement mirage. 9. Why is it that one can see the sun rise in the morning before it has really risen above the horizon? 10. What are the advantages of sealed-beam headlights? 11. Explain how traffic and driving lights are obtained in modern two-filament headlights. 12. Explain how glass can be rendered invisible. 13. Point out some of the applications of rendering glass invisible by the use of thin films. 14. Why is one likely to be deceived as to the depth of a pool of clear water? 15. How can one judge how far ahead a car is at night? 16. Define the terms: transparent, translucent, and opaque. 17. Differentiate between refraction and diffraction. 18. Differentiate between scattering and diffraction. 19. What evidence can you give to prove that light travels in a straight line?