The principle of superposition states that where two or more waves meet, the total displacement at any point is the vector sum of the displacements that each individual wave would cause at that point.
Two wave sources which are coherent have the same frequency (therefore same wavelength) and in phase or constant phase difference.
The effect of superposition of two coherent waves is interference.
Interference is the phenomenon when two or more waves overlap in the same region of space at the same time.
Superposition of waves which have same phase (2 crest or 2 trough) to produce a resultant wave which vibrates with maximum amplitude.
Superposition of waves which are anti-phase (crest and trough) to produce a resultant wave which vibrates with minimum amplitude.
An anti-node is a point of maximum amplitude where constructive interference occurs. Whereas a node is point of zero amplitude where destructive interference occurs.
The anti-node line joins all anti-node points. The node line joins all node points.