Question Video: Explaining Why a Resonant Cavity Uses a Less than Completely Reflective Face | Nagwa Question Video: Explaining Why a Resonant Cavity Uses a Less than Completely Reflective Face | Nagwa

# Question Video: Explaining Why a Resonant Cavity Uses a Less than Completely Reflective Face Physics • Third Year of Secondary School

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. [E] All the reasons given are true.

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### 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.

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