What contains the active medium and consists of a fully reflecting mirror at one end and a partially reflecting mirror at the other?

• A. Optical Cavity
• B. Power Pumping Station
• C. Gain Medium Chamber
• D. Emission Vessel

Answer: (A)

The concept here is an optical cavity, the resonator that contains the active (gain) medium and uses mirrors to trap and shape light. In a laser-style cavity, light is amplified by the gain medium, but to build up a strong, coherent beam it needs feedback. Two mirrors create that feedback: one is fully reflecting to keep most of the light inside, and the other is partially reflecting to let a portion escape as the usable laser output. This arrangement defines the resonant modes and lets the light circulate long enough to be amplified, so the output emerges from the partially reflecting end. Other terms don’t describe this feedback structure: they don’t specify the mirrored cavity that houses the active medium and supports standing-wave modes.