SM_chapter40 - 40 Introduction to Quantum Physics Note: In...

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40 Introduction to Quantum Physics CHAPTER OUTLINE 40.1 Blackbody Radiation and Planck’s Hypothesis 40.2 The Photoelectric Effect 40.3 The Compton Effect 40.4 Photons and Electromagnetic Waves 40.5 The Wave Properties of Particles 40.6 The Quantum Particle 40.7 The Double-Slit Experiment Revisited 40.8 The Uncertainty Principle ANSWERS TO QUESTIONS Q40.1 The f rst fl aw is that the Rayleigh–Jeans law predicts that the intensity oF short wavelength radiation emitted by a blackbody approaches inf nity as the wavelength decreases. This is known as the ultraviolet catastrophe . The second fl aw is the prediction oF much more power output From a blackbody than is shown experimentally. The intensity oF radiation From the blackbody is given by the area under the red IT λ , ( ) v s . l curve in ±igure 40.5 in the text, not by the area under the blue curve. Planck’s Law dealt with both oF these issues and brought the theory into agreement with the experimental data by adding an exponential term to the denominator that depends on 1 . This both keeps the predicted intensity From approaching inf nity as the wavelength decreases and keeps the area under the curve f nite. Q40.2 Our eyes are not able to detect all Frequencies oF electromagnetic waves. ±or example, all objects that are above 0 K in temperature emit electromagnetic radiation in the inFrared region. This describes everything in a dark room. We are only able to see objects that emit or refl ect electromagnetic radiation in the visible portion oF the spectrum. *Q40.3 (i) The power input to the f lament has increased by 8 × 2 = 16 times. The f lament radiates this greater power according to SteFan’s law, so its absolute temperature is higher by the Fourth root oF 16. It is two times higher. Answer (d). (ii) Wien’s displacement law then says that the wavelength emitted most strongly is halF as large: answer (F). Q40.4 No. The second metal may have a larger work Function than the f rst, in which case the incident photons may not have enough energy to eject photoelectrons. Q40.5 Comparing Equation 40.9 with the slope-intercept Form oF the equation For a straight line, ym xb =+ , we see that the slope in ±igure 40.11 in the text is Planck’s constant h and that the y intercept is φ , the negative oF the work Function. IF a diFFerent metal were used, the slope would remain the same but the work Function would be diFFerent. Thus, data For diFFerent metals appear as parallel lines on the graph. 433 Note : In chapters 39, 40, and 41 we use u to represent the speed oF a particle with mass, reserving v For the speeds associated with reFerence Frames, wave Functions, and photons.
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434 Chapter 40 Q40.6 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 sufF ciently high frequency for the effect to occur.
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This note was uploaded on 01/19/2012 for the course PHY 232 taught by Professor Williams,frank during the Spring '11 term at Ohio State.

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SM_chapter40 - 40 Introduction to Quantum Physics Note: In...

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