When a wave is reflected from a rigid boundary (fixed end of a string), the reflected wave undergoes

1. At a fixed (rigid) end, boundary condition requires zero displacement. 2. The reflected wave must cancel the incident at that point. 3. This forces a phase change of π on reflection from a rigid boundary. 4. At a free end (open string end), the reflection has NO phase change. _Source: syllabus Physics, Ch 14 "Waves", §14.6_