...
Coherence and Interference are two ideal wave features that complement each other.The wave interference do occur but for it to be considered constructive interference,it needs the coherence of some of the physical quantities of the waves involved; the wavelength, amplitude, and phase of the constituent wave forms must be identical.The most distinct concept of coherence in waves requires that the wave are in phase,their peaks are aligned with each other.
Interference
...
Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium.
The interference of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium.
...
Constructive interference is a type of interference that occurs at any location along the medium where the two interfering waves have a displacement in the same direction.
In this case, both waves have an upward displacement; consequently, the medium has an upward displacement that is greater than the displacement of the two interfering pulses. Constructive interference is observed at any location where the two interfering waves are displaced upward. But it is also observed when both interfering waves are displaced downward. This is shown in the diagram below for two downward displaced pulses.
...
In this case, a sine pulse with a maximum displacement of -1 unit (negative means a downward displacement) interferes with a sine pulse with a maximum displacement of -1 unit. These two pulses are drawn in red and blue. The resulting shape of the medium is a sine pulse with a maximum displacement of -2 units.
Destructive interference interference
----Destructive interference is a type of interference that occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction.
...