The diagram below shows a 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.
To determine if the stimulated emission is shown incorrectly, let’s look at all of the steps of stimulated emission from the beginning. Atoms typically have their electrons in the lowest energy level states possible. But in order for an emission to occur, an electron must first be in an excited state, which typically occurs when a photon is incident on an atom and contains enough energy such that it can be absorbed by the electron causing the electron to transition to a higher energy level. This is the first step of stimulated emission, starting with an excited electron.
Once this has been accomplished, another photon comes in and interacts with the already excited electron before it has time to decay. If the energy of this photon does not match the energy required to cause the already excited electron to transition up to a yet higher energy level but instead has an energy between the energy levels the electron originally transitioned through, then instead of being absorbed, the electron will be stimulated, which is where the name for the emission type comes from.
This stimulation causes the electron to transition down to a lower energy level, emitting a photon in the process. The photon that caused the simulation process travels in the same direction before and after interacting with the excited electron. And the photon that is emitted from the electron as it transitions down will also be traveling in the same direction. But it doesn’t stop there. These two photons will have the same energy and same phase as each other as well. Whenever there is a stimulated emission, we should expect to see two photons: the one that caused the stimulation and the one that is emitted from the electron. And both of these photons should have the same direction, phase, and energy.
From the diagram that we’re given, it should be clear that the two photons are not traveling in the same direction; one of them is at a bit of an angle. So the answer to “Which of the following ways is the stimulated emission incorrectly shown?” is (A) the photons move in different directions.