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Unformatted text preview: 35 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 Reflection 35.5 Refraction 35.6 Huygens Principle 35.7 Dispersion and Prisms 35.8 Total Internal Reflection 35.9 Fermats Principle The Nature of Light and the Laws of Geometric Optics ANSWERS TO QUESTIONS Q35.1 The ray approximation, predicting sharp shadows, is valid for << d . For ~ d diffraction effects become important, and the light waves will spread out noticeably beyond the slit. Q35.2 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 significant, but astronomers who look at other galaxies can gain an idea of what galaxies looked like when they were significantly younger. Thus, it actually makes sense to speak of looking backward in time. Q35.3 Sun Moon Note: Figure not at all to scale no eclipse partial eclipse Earths surface total eclipse (full shadow) no eclipse FIG. Q35.3 315 316 The Nature of Light and the Laws of Geometric Optics Q35.4 With a vertical shop window, streetlights and his own reflection can impede the window shoppers clear view of the display. The tilted shop window can put these reflections out of the way. Windows of airport control towers are also tilted like this, as are automobile windshields. FIG. Q35.4 Q35.5 We assume that you and the child are always standing close together. For a flat wall to make an echo of a sound that you make, you must be standing along a normal to the wall. You must be on the order of 100 m away, to make the transit time sufficiently long that you can hear the echo separately from the original sound. Your sound must be loud enough so that you can hear it even at this considerable 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 reflect from both walls with high efficiency to reverse in direction and come back to you. You can stand anywhere reasonably far away to hear a retroreflected echo of sound you produce. If the two walls are not perpendicular, the inside corner will not produce retroreflection. 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 retroreflects the sound, but the perpendicular walls on both sides of the corner. Diagram (b) applies also in this case....
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This homework help was uploaded on 04/13/2008 for the course PHYS 211 taught by Professor Shannon during the Spring '08 term at MSU Bozeman.
- Spring '08