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 full wavelength longer than the other wave. Because the waves were initially in phase, delaying one of them by one full wavelength means they are still in phase. This results in constructive interference. This is discussed on pages 549-551, and shown in Figure 29.12a. Delaying one wave by a wavelength doesn't change the fact that they are in phase.