Instructors_Guide_Ch24 - 24 Modern Optics and Matter Waves...

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24 Modern Optics and Matter Waves Recommended class days: 2 Background Information One of the goals of Physics for Scientists and Engineers is an early introduction of modern physics. We are in danger, in the introductory course, of teaching a subject that seems to have discovered nothing new in the last 200 years. It is important to convey to students at least some of the exciting ideas of 20th century physics. An early introduction of modern physics can awaken student interest in the course. This is also a good point to connect with the words and phrases that many students know from popular science literature or science fiction—photons, wave-particle duality, and so on. Most students don’t know anything substantive about these concepts. Modern physics ideas can be introduced in a natural, pedagogically meaningful way as a satisfying conclusion to Part V. Atomic spectra, matter waves, and quantization are all extensions of wave ideas into the atomic realm. Alternatively, this chapter can be skipped for now and later used as an introduction to Part VII. The focus is on experimental evidence for discrete spectra, for the interference and diffraction of matter, and for a particle-like behavior of light. This chapter does not present a theory or expla- nation for the observations. The information is intended to provide preliminary answers to several “How do we know …?” questions: how do we know about the structure of atoms, how do we know what light is, how do we know what happens in the atomic and subatomic world, and so on. The answers are “preliminary” in the sense that this chapter is intended to stir interest, not to be the final word on the subject. All of these issues will be taken up again in Part VII, at which time quantum physics will be introduced as a theoretical framework to understand the observations. As noted, many of the ideas are extensions of wave physics. In particular: • Atomic spectra are an application of diffraction gratings. • X-ray diffraction shows that the crystal structure of a solid acts as a diffraction grating. • The evidence for matter waves is an application of interference and diffraction. • Energy quantization is explained on the basis of standing waves. Photons, of course, are going back toward particle physics. But the double-slit experiment at low light levels makes it very clear that the “light particles” are certainly not particles in a classical sense. This chapter acquaints students with ideas that they’ll see again, but there is no expectation they will master the concepts at this point. The major points they should remember are: • Discrete spectra. • Photons. • Matter waves and the de Broglie wavelength. • Energy quantization as a consequence of confining a matter wave.
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Instructors_Guide_Ch24 - 24 Modern Optics and Matter Waves...

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