Teaching Physics with the Physics Suite

Edward F. Redish

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Absorbing and emitting photons

A quantum system (a molecule) has the energy levels shown in the diagram at the right. The energies are as follows:
  • E0 = 1.1 eV
  • E1 = 1.8 eV
  • E2 = 3.2 eV
We begin with a large number of these molecules in their ground states.

  1. We want to raise a lot of these molecules to the state labeled E2 by shining light on it. What wavelength of light should we use?
  2. If we expose our molecules to a continuous spectrum of light, photons of particularly energies might be absorbed. What energy photons would you expect to absorb? Explain.
  3. .. If we manage to successfully excite a number of the molecules to the state E2, they can decay by emitting photons. What energy photons might you expect to see? Explain.

Note to the instructor: Students run into the same problem here that they did at the beginning of the course: getting confused between the value of a quantity (here the energy) and a change in that quantity.

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Page last modified October 31, 2002: MP07