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Unformatted text preview: Physics 222, Modern Physics, Exam 1 NAME______________________________________ You are graded on your work, with partial credit where it is deserved. Please be clear and well-organized in all your steps. 1. Gold has a work function of 4.83 eV. (a) (6) What is the value of the threshold frequency? I.e., what is the lowest frequency for electromagnetic radiation incident on a gold surface if photoelectrons are to be emitted in the photoelectric effect? Answer ___________ (b) (6) Now suppose that ultraviolet light with a wavelength of 150 nm shines on a gold surface. What is the stopping potential? I.e., what is the potential of the collector relative to the emitter which will barely prevent electrons from making it to the collector and producing a current? Answer ___________ (c) (6) Using Einsteins theory of the photoelectric effect, roughly sketch a plot of eV versus f , where e is the fundamental charge, V is the stopping potential, and f is the frequency of the light. Then clearly explain how a plot like this of experimental data can be used to measure the value of the work function and the value of Plancks constant. 2. From a 2002 astronomy article: Using the Hubble Space Telescope, scientists looked at the emissions of excited triply ionized nitrogen atoms in planetary nebula NGC3918 (around a dying star), and observed a lifetime of 2500 seconds for one particular transition. This is the longest lifetime ever observed for an excited state of an atom. (a) (6) Using the uncertainly principle, estimate the width (i.e. uncertainty) in energy for the photon emitted during this transition, as an electron in the atom falls from the excited state to a lower energy state. Give your answer in eV....
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