Chapter 40 - 40 Introduction to Quantum Physics CHAPTER...

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40 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 Introduction to Quantum Physics ANSWERS TO QUESTIONS Q40.1 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. Q40.2 The first flaw is that the Rayleigh–Jeans law predicts that the intensity of short wavelength radiation emitted by a blackbody approaches infinity as the wavelength decreases. This is known as the ultraviolet catastrophe . The second flaw is the prediction much more power output from a black-body than is shown experimentally. The intensity of radiation from the blackbody is given by the area under the red IT λ , bg vs. curve in Figure 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 infinity as the wavelength decreases and keeps the area under the curve finite. Q40.3 Our eyes are not able to detect all frequencies of energy. For 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 reflect electromagnetic radiation in the visible portion of the spectrum. Q40.4 Most stars radiate nearly as blackbodies. Vega has a higher surface temperature than Arcturus. Vega radiates most intensely at shorter wavelengths. Q40.5 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. Q40.6 Comparing Equation 40.9 with the slope-intercept form of the equation for a straight line, ym xb =+ , we see that the slope in Figure 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. 461
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462 Introduction to Quantum Physics Q40.7 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.
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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.

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Chapter 40 - 40 Introduction to Quantum Physics CHAPTER...

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