A resonant cavity is a space designed to enhance specific frequencies of sound or electromagnetic waves. It typically consists of reflective surfaces that allow waves to bounce back and forth, reinforcing certain frequencies while diminishing others. This principle is used in various applications, including lasers and musical instruments. In the context of lasers, the resonant cavity contains mirrors that reflect light, creating a coherent beam. Similarly, in musical instruments, such as violins, the body acts as a resonant cavity, amplifying sound produced by the strings. This design is crucial for achieving desired acoustic or optical properties.