Super position principle:-

“According to this principle, when two or more waves travelling through a medium superimpose each other, a new wave is formed in which the resultant displacement of displacements due to in dividual waves   at that instant.”

Note :-

1. When two waves of same wavelength super impose on each other in phase :, then y = y₁ + y₂
2. When two waves of same wavelength superimpose on each other out of phase, then y = y₁ + y₂