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Question Video: Identifying Stimulated Emission of Photons Physics

The diagram shows stimulated emission of photons from an atom. In which of the following ways is the stimulated emission incorrectly shown? [A] The photons move in different directions. [B] The photons have different phases. [C] The emission is shown correctly.

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Video Transcript

The diagram below shows stimulated emission of photons from an atom. In which of the following ways is the stimulated emission incorrectly shown? (A) The photons move in different directions. (B) The photons have different phases. (C) The emission is shown correctly.

Let’s begin by briefly recalling some information about stimulated emission. Stimulated emission is a process in which a photon interacts with an excited electron, causing the electron to transition to a lower energy state. When the electron undergoes a downward energy level transition, energy must be transferred out of it, and this is done by it emitting a photon. In order for this to occur, the original photon that interacts with the excited electron must have an energy that is exactly equal to the difference between the electron’s initial and final energy levels. Then, after the original photon interacts with the electron, it simply continues on its way.

It’s also important to recall that the emitted photon is virtually identical to the original, interacting photon since it has the same energy, frequency, phase, and direction as the interacting photon.

Now, we’ve been told that this diagram is supposed to show this process. Thus, we can infer that this ground state electron was initially in an excited state, presumably this second energy level here. Then, a photon with the correct amount of energy came along and interacted with the electron, triggering stimulated emission. Thus, the electron emitted a photon and moved down to the ground state.

We can see two photons in the diagram, so we can deduce that one photon was the original interacting photon that stimulated the emission. That would be this photon here that’s farther from the atom than this other photon. This photon that’s closer to the atom must be the emitted photon, since it was emitted right after the first photon interacted with the electron. Notice that both of the photons are traveling in the same direction. This is what we expect to see in this scenario, and therefore we can eliminate option (A).

But option (B) suggests that the photons have different phases. If we look closely at the two photons, they do appear to have the same frequency, but notice that the second photon has a wave form that’s inverted, as compared to the first. It’s subtle, but we can see that this first photon has one great peak pointing upward and two big troughs pointing downward, yet the other photon has two great peaks and one big trough. Thus, the photons seem to be completely out of phase with each other, which does not occur in stimulated emission. Since we have identified an error in the diagram, answer option (C) can’t be true.

Therefore, our final answer is option (B). The diagram incorrectly shows stimulated emission because the photons have different phases.

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