The Principle of Superposition
This states that whenever two or more waves meet, the instantaneous total displacement at any point is equal to the vector sum of the individual displacements at that point.
The principle of superposition leads to interference between waves.
The wavesin blue andin red below are in phase. They have the same phase and wavelength. Adding the displacements at each point produces a larger waveThis is constructive interference.
The wavesandare equal in magnitude and opposite in sign (orout of phase). They are interfering destructively.
In general waves are not completely in or out of phase. A more complicated wave results when two waves are added, as below.