Final answer:
A wave reflecting off a fixed boundary does so with a 180° phase change, meaning a crest returns as a trough. The amplitude remains unchanged, in accordance with Newton's third law.
Step-by-step explanation:
Reflection from a Fixed Boundary
When a wave encounters a fixed boundary, it reflects with a 180° or π radians phase change, according to Newton's third law. This means if the incident wave had a crest approaching the fixed end, the reflected wave will come back as a trough and vice versa. Interference, wherein one wave passes through another, is distinct from reflection but also plays a role in wave interactions.
Regarding a fixed boundary condition, as the incident wave exerts an upward force on the boundary, the boundary applies an equal and opposite force, leading to the reflected wave being out of phase with the incident wave. In contrast, a free boundary condition causes the reflected wave to be in phase with the incident wave; the end of the string tied to a frictionless pole moves up and down accordingly, reflecting crests as crests and troughs as troughs.