Video Transcript
One of the reflecting faces of the
resonant cavity of a laser must be less than completely reflective for the laser to
be effective. Which of the following explains why
this is the case? (A) If both reflective faces of the
resonant cavity are equally reflective, light waves traveling in opposite directions
through the cavity destructively interfere. (B) External energy cannot be
supplied to the resonant cavity if both its reflective faces are perfectly
reflective. (C) Coherent light cannot be
emitted from the resonant cavity if both its reflective faces are perfectly
reflective. (D) Energy cannot be emitted from
the resonant cavity if both its reflective faces are perfectly reflective. Or (E) all the reasons given are
true.
Lasers produce coherent light,
which consists of many photons with a particular energy. These photons are produced by a
process called stimulated emission. Let’s remind ourselves how
stimulated emission works.
The active medium of a laser
contains many atoms, which each contain a number of electrons. If a photon passes through the
active medium, it can interact with an electron in one of the atoms and cause the
electron to emit another identical photon. This is called stimulated emission
because the first photon stimulates the electron to emit the second photon. This photon can cause further
stimulated emissions to occur, or it can become part of the laser beam.
To produce a laser beam, we need
many photons to be produced in this way, meaning we need lots of stimulated
emissions to occur. However, it is perfectly possible
for a photon to travel all the way through the active medium without interacting
with any electrons. If this happens, the photon will
not cause any stimulated emissions. This is where the resonant cavity
comes in.
To build the resonant cavity, we
simply place two mirrors, one on each side of the active medium. If a photon passes through the
active medium without causing any stimulated emission, it will reach the mirror at
the end and be reflected back. This means that the photon will
again pass through the active medium and have another chance to interact with an
electron in an atom. When the photon finally interacts
with an electron, it will stimulate the emission of another photon, which can also
be reflected back and forth until it causes another photon emission.
By reflecting photons back and
forth through the active medium, we produce more and more photons in total, creating
a stronger laser beam. This process is known as
amplification.
When we construct the resonant
cavity, one of the mirrors should be a full mirror, meaning it reflects every photon
that hits it. However, if both of the mirrors
were completely reflective, no photons would ever be able to leave the active
medium. This means that we would never see
any laser light. We need one of the mirrors to be a
partial mirror, which is not completely reflective. This means that some photons can
pass through the partial mirror and leave the active medium. The photons that can pass through
the partial mirror are what we see as a laser beam.
We’re now ready to answer this
question. If we look through our answer
options, we can see that option (C) lines up with what we’ve discussed so far. If both reflective faces are
perfectly reflective, then coherent light can’t be emitted from the resonant cavity,
which makes the laser pretty much redundant. To make sure this is correct, let’s
see if we can rule out some of the other options.
Option (A) is not the correct
answer, because the resonant cavity is designed in such a way that destructive
interference won’t occur. So, we can rule out option (A). Option (B) is not right, because
energy is normally supplied to the active medium using an electrical power source,
which we could still do, even if both mirrors of the resonant cavity were fully
reflective. So, we can eliminate option
(B). Since we’ve eliminated (A) and (B),
we can also rule out option (E), as we know that not all of these statements are
true.
Finally, let’s think about option
(D). Option (D) might look like a good
answer, because we know that we need photons to be emitted by the laser and that
photons are associated with energy. However, the purpose of a laser
isn’t really to simply emit energy. Lasers are used specifically to
produce coherent light, as mentioned in option (C). So, option (D) is not the best
choice to answer this question. This means that the correct answer
to this question is option (C). Coherent light cannot be emitted
from the resonant cavity if both its reflective faces are perfectly reflective.
