If two waves pass through the same point they collide and interfere with one another before continuing on with their separate journeys. There are two types of interference.

Constructive interference is where the two waves are in phase and combine to create a wave of greater amplitude.

Destructive interference is where the two waves are out of phase, or in anti-phase, and in effect cancel one another out to create a wave with an amplitude of zero.

Stationary waves

A stationary wave is created by two waves which both have the same frequency and are travelling in opposite directions to one another.