Chap 34 SM - Chapter 34 Wave-Particle Duality and Quantum...

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Chapter 34 Wave-Particle Duality and Quantum Physics Conceptual Problems 1 [SSM] The quantized character of electromagnetic radiation is observed by ( a ) the Young double-slit experiment, ( b ) diffraction of light by a small aperture, ( c ) the photoelectric effect, ( d ) the J. J. Thomson cathode-ray experiment. Determine the Concept The Young double-slit experiment and the diffraction of light by a small aperture demonstrated the wave nature of electromagnetic radiation. J. J. Thomson’s experiment showed that the rays of a cathode-ray tube were deflected by electric and magnetic fields and therefore must consist of electrically charged particles. Only the photoelectric effect requires an explanation based on the quantization of electromagnetic radiation. () c is correct. 2 •• Two monochromatic light sources, A and B, emit the same number of photons per second. The wavelength of A is λ A = 400 nm, and the wavelength of B is B = 600 nm. The power radiated by source B ( a ) is equal to the power of source A, ( b ) is less than the power of source A, ( c ) is greater than the power of source A, ( d ) cannot be compared to the power from source A using the available data. Determine the Concept Since the power radiated by a source is the energy radiated per unit area and per unit time, it is directly proportional to the energy. The energy radiated varies inversely with the wavelength ( hc E = ); i.e., the longer the wavelength, the less energy is associated with the electromagnetic radiation. b is correct. 3 [SSM] The work function of a surface is φ . The threshold wavelength for emission of photoelectrons from the surface is equal to ( a ) hc / , ( b ) / hf , ( c ) hf / , ( d ) none of above. Determine the Concept The work function is equal to the minimum energy required to remove an electron from the material. A photon that has that energy also has the threshold wavelength required for photoemission. Thus, = hf . In addition, f c = . It follows that = t hc , so hc = t and ( ) a is correct. 1
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Chapter 34 2 4 •• When light of wavelength λ 1 is incident on a certain photoelectric cathode, no electrons are emitted, no matter how intense the incident light is. Yet, when light of wavelength 2 < 1 is incident, electrons are emitted, even when the incident light has low intensity. Explain this observation. Determine the Concept We can use Einstein’s photoelectric equation to explain this observation. Einstein’s photoelectric equation is: φ = hf K max where , called the work function, is a characteristic of the particular metal. Because f = c / : = hc K max We’re given that when light of wavelength 1 is incident on a certain photoelectric cathode; no electrons are emitted independently of the intensity of the incident light. Hence, we can conclude that: 0 1 < hc < 1 hc When light of wavelength 2 < 1 is incident, electrons are emitted, electrons are emitted independently of the intensity of the incident light.
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This note was uploaded on 02/28/2012 for the course PHYSICS 122 taught by Professor Goussiou during the Spring '12 term at University of Washington.

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Chap 34 SM - Chapter 34 Wave-Particle Duality and Quantum...

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