How do interference happen ?
Superposition and Cancellation
When two identical waves pass each other, they can either have;
Constructive interference: when the 2 waves are in phase with each other, so the peaks and troughs superpose each other and double the amplitude
Destructive interference: This is the opposite of constructive, where the 2 waves are 180 degrees out of phase. As they're the same frequency, the 2 amplitudes are opposite, so cancel each other out creating a 'still' area