ism_ch27

# ism_ch27 - Chapter 27 Optical Instruments Answers to...

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186 Chapter 27 Optical Instruments Answers to Even-numbered Conceptual Questions 2. No. The lens will still show a complete image, though you may have to move your head more from side to side to see it all. 4. The reason things look blurry underwater is that there is much less refraction of light when it passes from water to your cornea than when it passes from air to your cornea. Therefore, your eyes simply aren’t converging light enough when they are in water. Since farsightedness is caused when your eyes don’t converge light as much as they should (see Figure 27-11), this can be considered as an extreme case of farsightedness. 6. A concave lens always forms an image smaller than the object, as can be seen in Figure 26-34. A convex lens, on the other hand, produces an enlarged image when the object is closer than the focal point, as shown in Figure 26-35 (b). Therefore, the person is wearing glasses that converge, like a convex lens. Such lenses are used to correct farsightedness (see Figure 27-11), and hence we conclude that the person is farsighted. 8. The answer is (b). If a person is nearsighted, the eye converges (bends) light too much to bring it to a proper focus on distant objects (see Figure 27-6). To reduce the amount of convergence, the intracorneal ring should decrease the cornea’s curvature – that is, it should make the cornea flatter. 10. Yes, it matters. A simple magnifier is nothing more than a convex lens. As we can see from Figure 26-35, a convex lens forms an enlarged (magnified) image only when the object is closer to the lens than its focal length. 12. As we can see in Figure 27-15 and Example 27-5, the lens with the shorter focal length is the one that is used as the objective. Therefore, we should pick the 0.45 cm lens to be the objective. 14. The instrument is a telescope. In general, the length of a telescope is roughly equal to the sum of the focal lengths of its objective and eyepiece, as we see in Figure 27-16. 16. The image you view when looking into a telescope is virtual. First, the objective forms a real image of a distant object, as shown in Figure 27-16. Next, the eyepiece forms an upright and enlarged image of the objective’s image. The situation with the eyepiece is essentially same as that shown in Figure 26-35 (b). Therefore, it is clear that the final image is virtual in this case. On the other hand, when a telescope is used to make a photograph, it can project a real image onto the photographic film. 18. As an object moves closer to the front of an octopus eye, the image it forms moves farther behind the eye. The situation is similar to that in Active Example 27-1 and Figure 26-35 (a). To keep the image on the retina, therefore, it is necessary to move the lens itself farther from the retina. 20. No. Chromatic aberration occurs in lenses because light of different frequency refracts by different amounts. In the case of a mirror, however, all light – regardless of its frequency – obeys the same simple law of reflection; namely, that the angle of reflection is equal to the angle of incidence. Since light of all colors is bent in the same way by a mirror, there is no chromatic aberration.

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