SAT Subject Physics Practice Question 141: Answer and Explanation

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Question: 141

4. The single electron in an atom has an energy of –40 eV when it’s in the ground state, and the first excited state for the electron is at –10 eV. What will happen to this electron if the atom is struck by a stream of photons, each of energy 15 eV?

A. The electron will absorb the energy of one photon and become excited halfway to the first excited state, then quickly return to the ground state, without emitting a photon.
B. The electron will absorb the energy of one photon and become excited halfway to the first excited state, then quickly return to the ground state, emitting a 15 eV photon in the process.
C. The electron will absorb the energy of one photon and become excited halfway to the first excited state, then quickly absorb the energy of another photon to reach the first excited state.
D. The electron will absorb two photons and be excited to the first excited state.
E. Nothing will happen.

Correct Answer: E

Explanation:

The gap between the ground-state and the first excited state is

–10 eV – (–40 eV) = 30 eV

Therefore, the electron must absorb the energy of a 30 eV photon (at least) to move even to the first excited state. Since the incident photons have only 15 eV of energy, the electron will be unaffected.

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