Summary: Physics 222, Modern Physics, Exam 1
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?
(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?
(c) (6) Using Einstein's theory of the photoelectric effect, roughly sketch a plot of eV0 versus f , where e is
the fundamental charge, V0 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 Planck's 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."