What is the difference between the mirrors in the Optical Cavity?

• A. One mirror is 100% reflective and the other is 95% reflective.
• B. Both mirrors are equally reflective at 50%.
• C. Both mirrors are fully reflective.
• D. One mirror is transparent.

Answer: (A)

In an optical cavity, light needs to be trapped long enough to build up a strong internal field, but you also want to be able to couple light into and out of the cavity. That’s achieved by using mirrors with different reflectivities. One mirror is kept nearly perfectly reflective to minimize losses and maintain the standing light inside. The other mirror is slightly less reflective so it transmits a small portion of the light, acting as the input/output coupler. This combination allows the light to enter, resonate, and then leak out in a controlled way.

If both mirrors were only about 50% reflective, the light would leak away too quickly and the cavity wouldn’t store much energy. If both were fully reflective, there would be little to no leakage, so there’d be no useful output. If one mirror were transparent, you’d lose the necessary reflection to sustain the cavity’s resonance.