They produce a spot that is bright.
Work Step by Step
One wave travels 2 full wavelengths longer than the other wave. Because the waves were initially in phase, delaying one of them by 2 full wavelengths 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 2 wavelengths doesn't change the fact that they are in phase.