Constructive interference can be seen at Wave Interference
It is the phenomenon that occurs when two waves meet while travelling along the same medium at the same time.
The waves pass through each other without being disturbed. If the two crests travelling in the same direction interfere with one another, then the resultant displacement will be greater than the individual displacement of each crest. This type of interference is known as Constructive interference.
If suppose the two waves have opposite displacements at the instant they pass one another, then the resultant displacement will be added to zero. This type of interference is known as Destructive interference.In general, whenever a number of waves come together the interference will not be completely constructive or completely destructive, but somewhere in between.
Phase is the term used to describe the relative positions of the crests and troughs. When the crests and troughs are aligned, for constructive interference, the two waves are said to be in phase. At points where destructive interference occurs, the waves are said to be completely out of phase.