The big idea: Flick one end of a long rope and a hump races to the far end — but the rope itself stays put, each bit only jerking up and down as the pulse goes by. A wave carries energy along while the particles just vibrate on the spot.
There are two ways a particle can vibrate compared with the wave's direction of travel: across it (transverse) or along it (longitudinal).
New words — define them now: Transverse wave: the particles vibrate perpendicular (at right angles) to the direction the wave travels. Example: light, and a wave on a rope.
Longitudinal wave: the particles vibrate parallel (back and forth along the same line) to the direction the wave travels. Example: sound.
Transverse (across)
- Particles move perpendicular to the wave's travel
- Shows crests (tops) and troughs (bottoms)
- Examples: light, water surface, a rope wave
Longitudinal (along)
- Particles move parallel to the wave's travel
- Shows compressions (squashed) and rarefactions (stretched)
- Examples: sound, a push-pull on a spring
More new words: In a longitudinal wave, a compression is where the particles are bunched close together (high pressure); a rarefaction is where they are spread apart (low pressure).
The one thing to hold on to: Ask: which way does a particle move compared with the wave?
At right angles → transverse. Back and forth along the same line → longitudinal.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
For any wave, one particle's displacement–time graph shows how that particle moves. From it you read the amplitude (the peak) and the period (one full cycle), and the period links to the frequency and wave speed through the data-booklet wave equation.
- wave speed (m s⁻¹)
- frequency — cycles per second (Hz)
- wavelength — one full cycle (m)
- period — time for one full cycle (s)
Which graph gives which length?: A displacement–time graph of one particle → read the period T (a time).
A displacement–distance snapshot of the whole wave → read the wavelength λ (a length).
Both look like the same wavy shape, so always check the x-axis label first.
Animated graph
Watch the graph build step by step in study mode.
A particle on a wave has a displacement–time graph that peaks at 4.0 cm and repeats every 4.0 s. Find the amplitude, the period and the frequency of the wave.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Study smarter, not longer
Most students waste 40% of study time on topics they already know. Our AI tracks your progress and optimizes every minute.
How this is tested — usually a quick one-mark Paper 1A read-off or deduction:
Paper 1A — read a graph
- Off a displacement–time graph of one particle, read the amplitude and frequency.
- Off a displacement–distance snapshot, read the wavelength.
Paper 1A — deduce motion
- Given a snapshot and how one point moves next, work out the wave's direction of travel and how another point is moving.
The classic trap: Confusing a snapshot (x-axis = distance → wavelength) with a displacement–time graph (x-axis = time → period).
The trick for 'which way is it moving?': On a transverse snapshot, a point takes on the displacement of the point just behind it (the side the wave came from).
So if the wave travels right, each point is about to copy the point on its left — look just to the left to see whether the point next goes up or down.
Animated graph
Watch the graph build step by step in study mode.
A transverse wave travels to the right. A snapshot (displacement–distance graph) shows the pattern repeating every 4.0 m, with a peak of 4.0 cm. Point P sits on the rising part of the curve, just to the right of a crest. Find the wavelength and state which way point P moves next.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.