An electron is excited from the ground state of an atom (energy level 1) into a highly excited state (energy level 8). Which of the following electron behaviors represents the fluorescence effect by the atom?
  1. The electron remains at level 8 for a very long time, then transitions up to level 9.
  2. The electron transitions directly down from level 8 to level 1.
  3. The electron transitions from level 8 to level 1 and then returns quickly to level 8.
  4. The electron transitions from level 8 to level 6, then to level 5, then to level 3, then to level 1.
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OpenStax College Physics for AP® Courses, Chapter 30, Problem 11 (Test Prep for AP® Courses)

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This is College Physics Answers with Shaun Dychko. Fluorescence means that the atom is emitting a wavelength that is visible and so that means the energy can't be too high otherwise the wavelength would be so short, it would be ultraviolet and we would not see it. So this is a very high energy level—8— and so a transition from 8 to 1 would have an energy that would create a photon of wavelength that is too short to see and so 8 to 1 is not possible. Transitioning from 8 to 9 doesn't even make sense because why would it go to a higher energy level yet. And then (c) doesn't make sense because well, 8 to 1 doesn't work because that will make too high of an energy and (d) is the answer. So it goes from 8 maybe to 6 then to 5 and then to 3 and then to 1 and possibly, from 6 to 5 would be a small-enough energy difference to create a long long enough wavelength such that we could see it and this might be their fluorescence is this transition from 6 to 5 maybe. The answer is (d).