What is the function of mirrors in a laser cavity?

Mirrors play a crucial role in the laser cavity by reflecting light back and forth between them. This repeated reflection allows the light to pass through the gain medium multiple times, which leads to amplification of the light. The gain medium, typically placed between the mirrors, is excited to produce photons. As these photons reflect off the mirrors, they stimulate further emission of photons from the gain medium, thereby increasing the intensity of the light.

The design of the laser cavity, including the configuration of the mirrors, ensures that the light is not only amplified but also properly directed. One mirror is fully reflective, while the other is partially reflective, allowing some of the amplified light to escape as the laser output. This mechanism of reflecting light back and forth is fundamental to producing a coherent and monochromatic beam of light, which are characteristic features of laser output.