The phase of a point on a wave describes where it is in its cycle of oscillation. The phase difference between two points (or two waves) tells you how far one is ahead of the other in the cycle.
Phase difference is measured as an angle. One complete cycle, which corresponds to one whole wavelength, is 360° or 2π radians.In phase: two points are moving in exactly the same way at the same time. They are separated by a whole number of wavelengths (phase difference 0, 2π, 4π ...).In antiphase: two points are moving in exactly opposite ways. They are separated by half a wavelength, or an odd number of half wavelengths (phase difference π, 3π ...).For two points a distance Δx apart along a wave of wavelength λ:phase difference = 2πΔx / λ (in radians) = 360° × Δx / λ The same idea works in time. If one oscillation lags another by Δt and the period is T, the phase difference is 2πΔt / T.Worked example: a wave has wavelength 0.80 m. Two points 0.20 m apart differ in phase by 2π × 0.20 / 0.80 = π/2 radians, which is 90°. One point reaches its peak a quarter of a cycle before the other.Phase difference matters whenever waves meet, because it decides whether they add together or cancel.
Phase difference is measured as an angle. One complete cycle, which corresponds to one whole wavelength, is 360° or 2π radians.In phase: two points are moving in exactly the same way at the same time. They are separated by a whole number of wavelengths (phase difference 0, 2π, 4π ...).In antiphase: two points are moving in exactly opposite ways. They are separated by half a wavelength, or an odd number of half wavelengths (phase difference π, 3π ...).For two points a distance Δx apart along a wave of wavelength λ: