When waves pass through each other they produce a phenomenon known as interference in which the amplitudes of the waves are added together to produce a resultant wave form.

When two crests overlap the resultant crest is higher (the amplitude increases), and when two troughs overlap the resultant trough is deeper (the amplitude increases). This is called constructive interference.

Constructive interference produces a resultant wave with a larger amplitude than the waves that combined to produce it.

When two or more wave sets combine to produce constructive interference, then the waves are said to be “in phase” with each other, and the amplitude increases.

Example: the waves below are in phase and show constructive interference

When waves are “out of phase” with each other, the crests overlap the troughs and destructive interference occurs.

Destructive interference produces a resultant wave with a smaller amplitude than the waves that combined to produce it.

Examples of constructive interference include:

Laser light (multiple waves of similar wavelength overlapping crest to crest and trough to trough)

Rogue waves (multiple sea waves of lower amplitude overlap crests to form one monstrous wave)

Some modern headphones have sound reduction technology that makes use of destructive interference to reduce the amplitude of incoming sound waves. A sensor monitors incoming sound waves (like those from jet engines) and a similar wave is produced that is perfectly out of phase with the incoming waves.

When two waves have almost the same wavelength, both constructive and destructive interference occur at regular intervals. This is known as the beat phenomenon, and can be used to tune musical instruments.