Phase difference measures how far one wave is ahead of another

Progressive Waves — AQA A-Level Physics

Key Definition

Phase difference — A measure of how much a point or a wave is in front of or behind another. Expressed in fractions of a wavelength, degrees or radians.

Phase differenceThe fraction of a cycle by which one wave leads or lags behind another, measured in degrees or radians. can be calculated between two points on the same wave, or the same point on two different waves.
When crests align with crests (or troughs with troughs), the waves are in phase.
When the crest of one wave aligns with the trough of another, they are in antiphase.

Key phase differences

In phase: phase differenceThe fraction of a cycle by which one wave leads or lags behind another, measured in degrees or radians. = 0, 360 degrees, or 2 pi radians (or any whole number of wavelengths).
In antiphase: phase $difference = 180 degrees$, or pi radians (or any odd number of half wavelengths).
Quarter wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). lead/lag: phase $difference = 90 degrees$, or pi/2 radians.

Converting between units

Fraction of lambda --> multiply by 360 degrees --> multiply by 2 pi for radians.
Example: 1/4 $\lambda = (1/4) x 360 = 90 degrees = (1/4) x 2 \pi = \pi/2 radians$.