When two or more waves simultaneously pass through the same medium, each wave acts on every particle of the medium, as if the other waves are not present. The resultant displacement of any particle is the vector addition of the displacements due to the individual waves.
Fig: Superposition principle
This is known as principle of superposition. If →Y1 and →Y2 represent the individual displacement then the resultant displacement is given by:
→Y = →Y1+ →Y2