The big idea: Watch ripples spread from where a raindrop hits a puddle. The expanding rings are wavefronts; an arrow pointing straight out from the centre is a ray.
Wavefront
- Each ring is a line of water all rising together
- Joins points all at the same point in their cycle (e.g. all the crests)
Ray
- The arrow shows which way the ripples head
- Shows the direction of travel
- Always drawn at right angles to the wavefronts
New word — phase: Phase means where a point is in its cycle — going up, at a crest, going down, at a trough.
Points in phase are doing exactly the same thing at the same time (e.g. two crests). A wavefront joins points that are all in phase.
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Two pictures of the same wave: Wavefronts = the crests, drawn as parallel lines · ray = an arrow pointing the way the wave goes, perpendicular to those lines.
For a wave spreading out from a point the wavefronts are circles; far away they look like straight, parallel lines (plane wavefronts).
Drawing wavefronts and rays
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The gap between two neighbouring wavefronts (crest to next crest) is exactly one wavelength λ. So the wavefront picture and the wave equation are the same idea seen two ways — the wave equation is given in the data booklet.
- wave speed — how fast the wavefronts move (m s⁻¹)
- frequency — wavefronts passing each second (Hz)
- wavelength — the gap between neighbouring wavefronts (m)
What each letter is on the picture: λ = the distance between two neighbouring wavefronts.
f = how many wavefronts pass a fixed point each second.
v = how fast the wavefronts move along the ray.
The wave equation as a triangle: v on top, f and λ underneath. Cover the one you want — the two side by side multiply, one above the other divides.
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Plane wavefronts on water are 0.50 m apart, and 3.0 of them pass a post each second. Find the speed of the wave.
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How this is tested — wavefronts and rays are mostly a representation you must read and draw:
Paper 1A
- Identify which line is a wavefront and which is a ray, and that the ray is perpendicular to the wavefronts.
Paper 1B / Paper 2
- Measure the wavelength from the wavefront spacing, then feed it into v = fλ.
- The same picture reappears in refraction (the wavefronts bend at a boundary).
The classic trap: The ray is NOT along a wavefront — it crosses them at 90°, pointing the way the wave travels.
Measuring λ off the picture: The wavelength is the gap between two neighbouring wavefronts, not the distance across several of them.
If the wavefronts sit at 2 m and 5 m, then λ = 5 − 2 = 3 m.
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A snapshot of a sound wave shows straight, parallel wavefronts. Neighbouring wavefronts are 3.0 m apart, and the source vibrates at 110 Hz. Find the speed of the sound.
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