---
title: "Progressive Waves: Phase and Path Difference"
description: "A progressive wave carries energy without carrying matter, and any two points on it differ by a phase measured in degrees or in wavelengths. Path difference then decides whether two waves arrive in st"
canonical: https://lightmysky.com/learn/science/progressive-waves-phase-and-path-difference-mt_RotpycCNF8
source: https://lightmysky.com/learn/science/progressive-waves-phase-and-path-difference-mt_RotpycCNF8.md
retrieved: 2026-09-12
---

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# Progressive Waves: Phase and Path Difference

A progressive wave carries energy without carrying matter, and any two points on it differ by a phase measured in degrees or in wavelengths. Path difference then decides whether two waves arrive in step.

Subject: Science · Area: Waves, Light & Sound · Ages 16 to 17
Page: https://lightmysky.com/learn/science/progressive-waves-phase-and-path-difference-mt_RotpycCNF8

## Ready when they can

- Reads amplitude, wavelength and period off displacement-distance and displacement-time graphs and says which graph is which
- States the phase difference between two points a stated fraction of a wavelength apart
- Turns a path difference into a phase difference and says whether the two waves arrive in phase

## Lesson: Reading waves and their phase gaps

A progressive wave carries energy without carrying matter. Read amplitude as the tallest point from rest, wavelength off a displacement against distance graph, and period off a displacement against time graph.

**Example.** Watch one floating gull as waves pass: it bobs up and down in place while the disturbance travels on. The distance between two neighbouring crests is one wavelength, and the time between two bobs is one period.

Two points differ in phase by their separation over wavelength times 360 degrees. A quarter wavelength means 90 degrees, and half a wavelength means antiphase at 180 degrees.

**Tip.** Whole wavelength gaps arrive in phase, crest to crest. Tell the graphs apart by their axes: distance plots show space at one instant, time plots show one place over time.

**Recap.** Read size off the right graph, and turn each gap into a phase.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Wave Equation](https://lightmysky.com/learn/science/the-wave-equation-mt_wmKhTiXt1p)

## Opens up

- [Superposition and Stationary Waves](https://lightmysky.com/learn/science/superposition-and-stationary-waves-mt_AIxA2TYGJN)
- [Intensity: the Energy and Power Carried by a Wave](https://lightmysky.com/learn/science/intensity-the-energy-and-power-carried-by-a-wave-mt_Hj-yG6g3CD)
- [The Postulates of Special Relativity and the Loss of Simultaneity](https://lightmysky.com/learn/science/the-postulates-of-special-relativity-and-the-loss-of-simultaneity-mt_kZqVQ18bbc)
- [Polarisation as Evidence for Transverse Waves](https://lightmysky.com/learn/science/polarisation-as-evidence-for-transverse-waves-mt_mCbxv77qDA)
- [The Wave Equation and Its Travelling-Wave Solutions](https://lightmysky.com/learn/science/the-wave-equation-and-its-travelling-wave-solutions-mt_Ue-v1cSxNx)