SM_PDF_chapter28 - Quantum Physics CHAPTER OUTLINE 28.1...

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771 Quantum Physics CHAPTER OUTLINE 28.1 Blackbody Radiation and Planck’s Theory 28.2 The Photoelectric Effect 28.3 The Compton Effect 28.4 Photons and Electromagnetic Waves 28.5 The Wave Properties of Particles 28.6 The Quantum Particle 28.7 The Double-Slit Experiment Revisited 28.8 The Uncertainty Principle 28.9 An Interpretation of Quantum Mechanics 28.10 A Particle in a Box 28.11 The Quantum Particle Under Boundary Conditions 28.12 The Schrödinger Equation 28.13 Tunneling Through a Potential Energy Barrier 28.14 Context Connection The Cosmic Temperature ANSWERS TO QUESTIONS Q28.1 Planck made two new assumptions: (1) molecular energy is quantized and (2) molecules emit or absorb energy in discrete irreducible packets. These assumptions contradict the classical idea of energy as continuously divisible. They also imply that an atom must have a definite structure—it cannot just be a soup of electrons orbiting the nucleus. Q28.2 (c) UV light has the highest frequency of the three, and hence each photon delivers more energy to a skin cell. This explains why you can become sunburned on a cloudy day: clouds block visible light and infrared, but not much ultraviolet. You usually do not become sunburned through window glass, even though you can see the visible light from the Sun coming through the window, because the glass absorbs much of the ultraviolet and reemits it as infrared. Q28.3 No. The second metal may have a larger work function than the first, in which case the incident photons may not have enough energy to eject photoelectrons. Q28.4 The Compton effect describes the scattering of photons from electrons, while the photoelectric effect predicts the ejection of electrons due to the absorption of photons by a material. Q28.5 Wave theory predicts that the photoelectric effect should occur at any frequency, provided the light intensity is high enough. However, as seen in the photoelectric experiments, the light must have a sufficiently high frequency for the effect to occur. Q28.6 A few photons would only give a few dots of exposure, apparently randomly scattered. Q28.7 The x-ray photon transfers some of its energy to the electron. Thus, its frequency must decrease. Q28.8 Light has both classical-wave and classical-particle characteristics. In single- and double-slit experiments light behaves like a wave. In the photoelectric effect light behaves like a particle. Light may be characterized as an electromagnetic wave with a particular wavelength or frequency, yet at the same time light may be characterized as a stream of photons, each carrying a discrete energy, hf . Since light displays both wave and particle characteristics, perhaps it would be fair to call light a “wavicle”. It is customary to call a photon a quantum particle, different from a classical particle.
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772 Quantum Physics Q28.9 An electron has both classical-wave and classical-particle characteristics. In single- and double-slit diffraction and interference experiments, electrons behave like classical waves. An electron has mass and charge. It carries kinetic energy and momentum in parcels of definite size, as classical particles
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This note was uploaded on 10/18/2008 for the course PHYS 3Q2341234 taught by Professor Dafsf during the Spring '08 term at UCLA.

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SM_PDF_chapter28 - Quantum Physics CHAPTER OUTLINE 28.1...

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