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.
What a learner can do afterwards
- 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
1 · Read
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.
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.
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.
Read size off the right graph, and turn each gap into a phase.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.