Teaching Physics with the Physics Suite

Edward F. Redish

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Interference with photons and electrons

If you put a laser beam through two very narrow slits, you see an interference pattern that looks something like the figure on the right. For the questions below give all the answers that are true. If none are true, write "N".
A) If you cover one of the slits, what will happen to the pattern?

  1. The intensity of the lines will decrease, but the pattern will remain.
  2. The lines on the same side as the open slit will remain, but the lines on the same side as the closed slit will disappear.
  3. The lines on the opposite side of the open slit will remain, but the lines on the side opposite to the closed slit will disappear.
  4. The pattern will disappear and the region shown will be almost uniformly bright.
  5. Something else will happen. (Describe it briefly.)
B) If you reduce the intensity of the laser beam enough so that only a single photon is passing through the system at a time — say putting one photon per second through the slits — which of the following statements are correct?
  1. There will be no change in the pattern and you will see it right away (as soon as the first photon hits the screen).
  2. The pattern will only be clear after a large number of photons have passed through the system.
  3. The pattern will not appear. A photon cannot interfere with itself.
  4. The pattern will appear because the photons know about the other photons that came before and will come later.
  5. Something else will happen. (Describe it briefly.)

C) Instead of using single photons, we send a beam of electrons through the double slits, one at a time. Which of the following statements are true?
  1. We will never see a pattern like the one shown, since electrons are particles, not waves.
  2. The pattern will only be clear after a large number of electrons have passed through the system.
  3. We will see an interference pattern, but only if we send a lot of electrons through at the same time.
  4. We will see an interference pattern, and how far apart it is spread will be independent of the energy of the electrons.
  5. We will see an interference pattern, and how far apart it is spread will depend on the energy of the electrons.
  6. None of these statements are true.

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Page last modified October 31, 2002: MP05