Work Step by Step
If no phase change occurs upon any of the reflections, then the situation is effectively one in which one wave travels a half-wavelength longer than the other wave. Because the waves were initially in phase, delaying one of them by a half-wavelength means they are now exactly out of phase. This results in destructive interference. This is discussed on pages 549-551, and shown in Figure 29.12b. Waves that are a half-wavelength out of phase cancel each other completely.