Constructive and destructive interference.

 

There are 2 types of interference:

interference_constructive_destructive.jpg Link to Video File  Click on the camera for a video

Constructive is when 2 waves with the same sign on their amplitude merge to create a larger wave. Think about when you 'construct' a building, you build it up just like the waves are going up.

 

Destructive is when 2 waves with opposite signs on their amplitude merge to reduce their amplitude. Think about destruction, you brake things down to smaller pieces. The wave here is being made smaller.