Superposition of waves When two waves pass through the same point they combine together to either constructively interfere with each other or destructively interfere with each other before passing on past each other and continuing their separate journeys. Constructive interference The two waves are in phase with each other and constructively interfere to give a wave of greater amplitude. 
Destructive interference The two waves are out of phase (anti-phase) with each other and destructively interfere to give a wave of zero amplitude.
Stationary waves Stationary waves are formed by two waves with the same frequency travelling in opposite directions.
