Video Transcript
Which of the following transitions
in an atom that is part of the active medium of a laser contribute to the production
of amplified coherent light? (A) Transition I and transition II
only, (B) all of the transitions shown, (C) transitions II, III, and IV only.
Let’s recall that in order to
obtain amplified coherent light, the basis of a laser beam, we want to create
stimulated emissions. Let’s start by analyzing each
transition and determine whether each one will help stimulated emission occur.
Looking at the diagram, we can see
that transition I corresponds to a spontaneous emission. This is when an electron goes from
the excited state to the ground state. To look at these energy transitions
another way, let’s consider a different type of diagram, which shows the relative
energies of the electrons in the atom. Now then, spontaneous emission
occurs so rapidly that an electron will only remain in the excited energy state for
about 10 nanoseconds, before decaying to the ground state. Since this process is so fast, it
is unlikely that an incident photon of the correct energy will reach the excited
electron and cause a stimulated emission before the electron decays back to the
ground state. This transition does not contribute
to the production of amplified coherent light.
Transition II occurs when we excite
the atom from the ground state to the excited state. When an electron is in an excited
state like this, an incident photon can cause the electron to undergo a stimulated
emission, where it releases a photon and descends to a lower energy level. The emitted photon will have the
same energy, direction, and phase as the photon that caused the emission. This means that both photons can
contribute to a beam of laser light. So this transition does contribute
to the production of amplified coherent light.
Let’s look at transition III. This transition is when the
electron goes from the excited state to the metastable state. An electron in the metastable state
will stay there, probably until it interacts with a photon of energy 𝛥𝐸 sub
𝐴. If this happens, then a stimulated
emission will occur, which means it can contribute to the production of an amplified
coherent light beam.
Now let’s see about transition
IV. The electron here starts in the
metastable state, where it will likely stay until it interacts with a photon of
energy 𝛥𝐸 sub 𝐴. This interaction will cause the
electron to undergo a stimulated emission, releasing another photon of energy 𝛥𝐸
sub 𝐴, with the same direction and phase as the incident photon. So this transition also contributes
to the amplified coherent beam.
Finally, looking at transition V,
we have to consider that it is unlikely. When an electron in a metastable
state absorbs a photon, it usually undergoes a stimulated emission rather than
becoming excited. In this case, if it does become
excited, it essentially will set up the scenario that we saw in transition I, where
an excited electron decays back to the ground state. This means, just like transition I,
transition V will not contribute to the creation of an amplified coherent beam.
With all of these transitions
covered, let’s now see. Transitions II, III, and IV
contribute to the production of amplified coherent light, while transitions I and V
do not. This means that the answer to which
transitions contribute to the amplified coherent beam in the active medium of a
laser are transitions II, III, and IV. Option (C) is correct.
