What is defined as the change when light or sound waves bounce backwards off a boundary?

The correct response to the question is that reflection is defined as the change that occurs when light or sound waves bounce backward off a boundary. This process happens when the waves encounter a surface that they cannot pass through, causing them to return into the medium from which they originated.

In the case of light, typical examples include a mirror reflecting light or a calm body of water reflecting the scenery above it. For sound, reflection can be observed in echoing sounds in a canyon or a large empty room. The angle at which the wave strikes the boundary (the angle of incidence) is equal to the angle at which it reflects away from the boundary (the angle of reflection), adhering to the law of reflection.

Understanding reflection is crucial in various applications, including optics, acoustics, and even technology like radar and sonar, where the principle of waves bouncing off surfaces is utilized to detect objects.