Final answer:
Two identical sine waves with a phase difference of 180° result in pure destructive interference, where the waves cancel each other out, leading to a resultant amplitude of zero.
Step-by-step explanation:
When two identical sine waves are traveling in the same direction with a phase difference of 180°, they are said to be out of phase. In this scenario, when the waves meet, one wave is at its peak (crest) while the other is at its lowest point (trough). This alignment of crest to trough results in pure destructive interference, meaning that the disturbances cancel each other out. Therefore, the resultant amplitude of these two waves when they interfere destructively is zero, because they are in complete opposite phases at every point along the waves (crest meets trough).