The phase difference of π radians that you found in the previous part provides a criterion for destructive interference. what phase difference corresponds to completely constructive interference (i. e., the original wave and the shifted wave coincide at all points)?

Answer and Explanation:

The phase difference corresponding to the constructive interference would be zero and that to the destructive interference would also be zero.

At the point when two waves meet so that their peaks line up together, at that point it's called constructive interference. The subsequent wave has a higher sufficiency.  

In destructive interference, the peak of one wave meets the trough of another, and the outcome is a lower amplitude.

i don't understand. explain it clearly