Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Figure 27.11 The amplitudes of waves add. (a) Pure constructive interference is obtained when identical waves are in phase. (b) Pure destructive interference occurs when identical waves are exactly out of phase, or shifted by half a wavelength. Answer: in phase
Output:
Pure constructive interference and pure destructive interference are distinguished by whether or not identical waves are what?