Final answer:
Cathy hears her voice echoed in a tunnel due to the reflection of sound waves off the tunnel walls, a property of waves affected by the acoustic impedance of the medium.
Step-by-step explanation:
When Cathy yells in a tunnel and hears her voice as an echo, this is an example of reflection. In this process, the sound waves that are emitted by Cathy's voice hit the tunnel walls and are reflected back toward her, allowing her to hear the sound again. This occurrence demonstrates a very useful property of waves. When a wave encounters the boundary of a medium, part of the wave can be reflected. The echo Cathy hears is the sound waves bouncing back from the solid surfaces of the tunnel without significant absorption or transmission through the material. This happens because the tunnel's walls have a large difference in acoustic impedance compared to the air inside the tunnel, resulting in most of the sound being reflected rather than absorbed or transmitted.
Reflection and transmission of waves are phenomena that can be observed in various contexts, not only with sound but also with electromagnetic waves. Different materials will reflect, absorb, or transmit waves differently based on their properties and the frequency of the waves. For instance, glass is transparent to visible light but opaque to ultraviolet radiation. This illustrates that the behavior of a wave when it encounters a boundary depends upon the characteristics of the medium and the wave itself.