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Definition of Destructive Interference:
Interference occurs when two or more waves meet while traveling through the same medium. There are two types of interference that could take place at any given location along the medium. If at a given location, both interfering waves are displaced in the opposite direction (one is up and the other down), then destructive interference is said to occur at that location.
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There is no shortcut for this question. It involves a lot of patience. Starting at location A, inspect the two individual pulses along that line to determine if their displacements from the rest position (bold line) are both up, both down, or one up and one down. Once determined, apply the definition of destructive interference to determine if that location is an example of destructive interference (see Dictionary section). Repeat the process for each of the locations.
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Many students understand that destructive interference occurs at a location where a crest meets a trough. Yet they miss this question because they have the wrong conception that the size of the crest must be the same as the size of the trough. But don't be fooled! The size of the crest and trough do not need to be the same in order for it to be considered an example of destructive interference. While there may not be complete destructive interference, it is still destructive interference if one wave is up and the other wave is down at a given location.
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