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SM_chapter35 - 35 The Nature of Light and the Laws of...

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35 The Nature of Light and the Laws of Geometric Optics CHAPTER OUTLINE 35.1 The Nature of Light 35.2 Measurements of the Speed of Light 35.3 The Ray Approximation in Geometric Optics 35.4 The Wave Under ReF ection 35.5 The Wave Under Refraction 35.6 Huygens’s Principle 35.7 Dispersion 35.8 Total Internal ReF ection ANSWERS TO QUESTIONS Q35.1 Light travels through a vacuum at a speed of 300 000 km per second. Thus, an image we see from a distant star or galaxy must have been generated some time ago. For example, the star Altair is 16 light-years away; if we look at an image of Altair today, we know only what was happening 16 years ago. This may not initially seem signi± cant, but astronomers who look at other galaxies can gain an idea of what galaxies looked like when they were signi± cantly younger. Thus, it actually makes sense to speak of “looking backward in time.” *Q35.2 10 4 m /(3 × 10 8 m /s) is 33 µ s . Answer (c). *Q35.3 We consider the quantity l / d . The smaller it is, the better the ray approximation works. In (a) it is like 0.34 m/1 m 0.3. In (b) we can have 0.7 m/2 mm 0.000 3. In (c), 0.4 m/2 mm 0.000 2. In (d), 300 m/1 m 300. In (e) 1 nm/1 mm 0.000 001. The ranking is then e, c, b, a, d. Q35.4 With a vertical shop window, streetlights and his own refl ection can impede the window shopper’s clear view of the display. The tilted shop window can put these refl ections out of the way. Windows of airport control towers are also tilted like this, as are automobile windshields. 295 FIG. Q35.4
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296 Chapter 35 Q35.5 We assume that you and the child are always standing close together. For a at wall to make an echo of a sound that you make, you must be stand- ing along a normal to the wall. You must be on the order of 100 m away, to make the transit time suf± ciently long that you can hear the echo sepa- rately from the original sound. Your sound must be loud enough so that you can hear it even at this consider- able range. In the picture, the dashed rectangle represents an area in which you can be standing. The arrows represent rays of sound. Now suppose two vertical perpendicular walls form an inside corner that you can see. Some of the sound you radiate horizontally will be headed generally toward the corner. It will refl ect from both walls with high ef± ciency to reverse in direction and come back to you. You can stand anywhere reasonably far away to hear a retrorefl ected echo of sound you produce. If the two walls are not perpen- dicular, the inside corner will not produce retrorefl ection. You will generally hear no echo of your shout or clap. If two perpendicular walls have a reasonably narrow gap between them at the corner, you can still hear a clear echo. It is not the corner line itself that retrorefl ects the sound, but the perpendicular walls on both sides of the corner. Diagram (b) applies also in this case.
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SM_chapter35 - 35 The Nature of Light and the Laws of...

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