Wave path difference

If we have two waves that are in phase at a certain point, the path difference is the difference in the distance that the two waves have traveled to reach that point.

We measure it in terms of the wavelength () of the waves.

Constructive interference

If the path difference is a multiple of the wavelength (for example, , , ), the waves will interfere constructively, and create a wave with a larger amplitude.

Destructive interference

If the path difference is a half multiple of the wavelength (for example, , , ), the waves will interfere destructively, and create a wave with a smaller amplitude (or even zero if the waves had the same amplitude to start with).