Sample Problem

Teaching Physics with the Physics Suite

Edward F. Redish

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Getting a phototube to work

Monochromatic light of wavelength λ is shone on a photoelectric tube (metal cathode in a vacuum tube) as shown in the figure at the right. The metal in the cathode has a workfunction with a value of φ. As set up, the light fails to produce any photoelectrons, even when there is no retarding voltage placed on the tube. In order to try to get photoelectrons out, it would be appropriate to try the following: (Give all answers that are correct.)
  1. Replace the filter by one that passes a longer wavelength.
  2. Replace the filter by one that passes a shorter wavelength.
  3. Replace the metal cathode by one with a larger workfunction.
  4. Replace the metal cathode by one with a smaller workfunction.
  5. Increase the retarding potential a bit.
  6. Decrease the retrading potential a bit.
  7. There is nothing you can do to knock photoelectrons out.
Explain your reasoning.

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Page last modified May 10, 2006: MP06