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Unformatted text preview: CHAPTER 29 PARTICLES AND WAVES ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (a) At the higher temperature, the intensity per unit wavelength is greater, and the aximum occurs at a shorter wavelength (see Section 29.2). 2. (b) An X-ray photon has a much greater frequency than does a microwave photon (see Section 24.2). The X-ray photon also has a greater energy E , because E = hf (Equation 29.2), where h is Plancks constant and f is the frequency. The wavelength and frequency of a photon are related by = c/f (Equation 16.1), where c is the speed of light in a vacuum. Since the microwave photon has the smaller frequency, it has the greater wavelength. 3. (c) A green photon has a greater frequency than does a red photon (see Section 24.2). Therefore, the green photon possesses a greater energy E , because E = hf (Equation 29.2), where h is Plancks constant and f is the frequency. 4. (e) As the wavelength of a photon becomes smaller, its frequency and its energy become larger (see Section 29.3). Since the energy of an incident photon is equal to the maximum kinetic energy of the photoelectron plus the work function of the metal, increasing the incident energy increases the maximum kinetic energy of the photoelectron. The work function is a characteristic of the metal, and does not depend on the photon. 5. (d) The maximum kinetic energy of the ejected photoelectrons depends on the energy of the incident photons. Doubling the number of photons per second that strikes the surface does not change the energy of the incident photons, which depends only on the frequency of the photons. Thus, the maximum kinetic energy of the ejected photoelectrons does not change. However, doubling the number of photons per second that strikes the surface means that twice as many photoelectrons per second are ejected from the surface. 6. (b) Whether or not electrons are ejected from the surface of the metal depends on the energy of the incident photons and the work function of the metal The work function depends on the type of metal (e.g., aluminum or copper) from which the plate is made. (See Section 29.3). 7. W = 3.6 10- 19 J 8. KE max = 2.7 10- 20 J 9. (d) According to the discussion in Section 29.4, a photon has a momentum whose magnitude p is related to its wavelength by p = h / . 231 PARTICLES AND WAVES 10. (b) In the Compton effect, an X-ray photon strikes an electron and, like two billiard balls (particles) colliding on a pool table, the X-ray photon scatters in one direction and the electron recoils in another direction after the collision. 11. (a) In the Compton effect, some of the energy of the incident photon is given to the recoil electron. Therefore, the energy of the scattered photon is less than that of the incident photon. Since the energy of a photon depends inversely on its wavelength (see Section 29.3), the wavelength of the scattered photon is greater than that of the incident photon....
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