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.