Ch38 - Chapter 38 Photons and Matter Waves The sub-atomic...

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1 Chapter 38 The sub-atomic world behaves very differently from the world of our ordinary experiences. Quantum physics deals with this strange world and has successfully answered many questions in the sub-atomic world, such as: Why do stars shine? Why do elements order into a periodic table? How do we manipulate charges in semiconductors and metals to make transistors and other microelectronic devices? Why does copper conduct electricity but glass does not? In this chapter we explore the strange reality of quantum mechanics. Although many topics in quantum mechanics conflict with our common sense world view, the theory provides a well-tested framework to describe the sub- atomic world. Photons and Matter Waves 38-
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2 Quantum physics: •Study of the microscopic world • Many physical quantities found only in certain minimum (elementary) amounts, or integer multiples of those elementary amounts •These quantities are "quantized" •Elementary amount associated with this a quantity is called a "quantum" (quanta plural) Analogy example: 1 cent or $0.01 is the quantum of U.S. currency Electromagnetic radiation (light) is also quantized, with quanta called photons. This means that light is divided into integer number of elementary packets (photons). The Photon, the Quantum of Light 38-
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3 The energy of light with frequency f must be an integer multiple of hf . In the previous chapters we dealt with such large quantities of light, that individual photons were not distinguishable. Modern experiments can be performed with single photons. So what aspect of light is quantized? Frequency and wavelength still can be any value and are continuously variable, not quantized The Photon, the Quantum of Light, cont'd 38- (photon energy) Eh f = c f λ = However, given light of a particular frequency, the total energy of that radiation is quantized with an elementary amount (quantum) of energy E given by: Where the Planck constant h has a value: 34 15 6.63 10 J s 4.14 10 eV s h −− where c is the speed of light 3x10 8 m/s
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5 When short wavelength light illuminates a clean metal surface, electrons are ejected from the metal. These photoelectrons produce a photocurrent. First Photoelectric Experiment: Photoelectrons stopped by stopping voltage V stop . The kinetic energy of the most energetic photoelectrons is The Photoelectric Effect 38- Fig. 38-1 max stop K eV = K max does not depend on the intensity of the light! single photon ejects each electron
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6 Demo 38.1: The Photoelectric Effect 38- Photon (E γ , p γ ) Electron (E e , p e ) W (work function) electron
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7 The Photoelectric Effect, cont’d 38- Fig. 38-2 Second Photoelectric Experiment: Photoelectric effect does not occur if the frequency is below the cutoff frequency f o , no matter how bright the light! single photon with energy greater than work function Φ ejects each electron slope ab bc =
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8 The Photoelectric Effect, cont’d 38- Photoelectric Equation The previous two experiments can be summarized by the following equation, which also expresses energy conservation max (photoelectric equation) hf K =+ Φ max
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This note was uploaded on 04/08/2008 for the course PHY 207 taught by Professor Berim during the Spring '08 term at SUNY Buffalo.

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Ch38 - Chapter 38 Photons and Matter Waves The sub-atomic...

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