Phase

Constructive interference

If two waves coincide with peaks and troughs matching they are said to be in phase.

Two waves coinciding with peaks and troughs to match.

If two periodic waves of similar frequency coincide in phase the waves superimpose their wave energy to produce a wave of double the amplitude. This is constructive interference.

curriculum-key-fact
In phase = constructive interference
Two waves coinciding with peaks and troughs to match and + symbol between them. Below the two waves is wave with the peak and trough heights intensified by two. Between the two peaks is an = symbol.

Destructive interference

If two waves coincide with peaks of one meeting troughts of the other they are said to be out of phase.

If two waves are exactly out of phase they will interfere destructively to produce zero amplitude.

Two waves out of phase peak close together, top wave peak and bottom wave trough very far apart. + symbol between waves. Right hand side has = symbol and then a flat line.

curriculum-key-fact
Out of phase = destructive interference