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AQA GCSE Physics · Topic 4.6 Waves
Transverse and Longitudinal Waves: Amplitude, Wavelength, Period and Frequency
In this lesson you'll learn to: describe the difference between transverse and longitudinal
waves and give examples of each, explain why a wave transfers energy but not matter, define
amplitude, wavelength, period and frequency and read them off a graph, and use
f = 1 ÷ T.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, switch between Transverse and Longitudinal and watch the gold particle: does it ever travel along with the wave? On tab 2, make the light brighter and dimmer and watch which part of the wave changes. Then look at the x-axis on tabs 3 and 4: one says distance, one says time.
Watch the gold one. It only moves back and forth about the same spot. It never travels along with the wave. A wave transfers energy, not matter.
Amplitude = the maximum displacement of a point on the wave from its rest position. Measure from the middle line up to a peak, never from a trough to a peak.
Wavelength
Check the x-axis first. If it says distance, the graph is a photo of the whole wave at one instant, and peak to next peak is the wavelength. If it says time, it is the period (tab 4).
Frequency
f =
Hz
Period T = the time for one complete wave to pass a point. Frequency f = the number of waves passing a point each second, in hertz (Hz). f = 1 ÷ T, with T in seconds.
Slowed right down so you can see it. Real sound waves vibrate hundreds of times a second.
▲ Everything is slowed right down. Four tabs: wave types, amplitude, wavelength, then period and frequency.
Seen the gold particle stay put? Now learn the words.
Every mark on these two workbook pages comes from five key words and four definitions. They're all below, in the exact wording the mark scheme wants.
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Revision notes
Waves transfer energy, not matter
When a wave passes, the particles oscillate (vibrate) about a fixed point. They do not travel with the wave. What travels is the energy.
⭐ The evidence the exam asks for
Ripples on water: a cork or a leaf on the surface bobs up and down as the ripples pass, but it does not move along with them. So the water itself isn't travelling.
Sound in air: the air from a loudspeaker doesn't travel across the room to you (there's no wind from the speaker). The air particles just vibrate backwards and forwards and pass the energy on.
Transverse and longitudinal
Every wave is one or the other. It depends on which way the particles oscillate compared with the direction of energy transfer (the way the wave travels).
↕
Transverse
The oscillations are perpendicular to the direction of energy transfer.
Has: peaks (crests) and troughsExamples: water ripples, light and all electromagnetic waves, a slinky shaken side to side
↔
Longitudinal
The oscillations are parallel to the direction of energy transfer.
Has: compressions and rarefactionsExamples: sound, ultrasound, infrasound, a slinky pushed and pulled
| Word | What it means |
|---|---|
| Compression (C) | where the particles are close together |
| Rarefaction (R) | where the particles are spread far apart |
| Ultrasound | sound with a frequency above 20 000 Hz, too high for humans to hear |
| Infrasound | sound with a frequency below 20 Hz, too low for humans to hear |
⭐ The five key words
Every answer about wave types uses these: oscillation, perpendicular, parallel, direction, energy transfer. "Up and down" or "side to side" on their own are not enough.
Amplitude, wavelength, period and frequency
| Property | Definition (learn it word for word) | Unit |
|---|---|---|
| Amplitude | the maximum displacement of a point on the wave from its rest position (the undisturbed position) | m, cm |
| Wavelength λ | the distance from one point on a wave to the same point on the next wave, e.g. crest to crest | m, cm |
| Period T | the time taken for one complete wave to pass a point | s |
| Frequency f | the number of waves passing a point each second | hertz, Hz |
⭐ Amplitude: middle to top, never top to bottom
The amplitude arrow starts on the rest position (the middle line) and stops at a crest. Peak to trough is twice the amplitude, and it's the most common way to lose this mark.
A bigger amplitude means the wave carries more energy: brighter light, a louder sound. Amplitude zero is a flat line: no wave at all.
⭐ Check the x-axis before you read anything
x-axis = distance: the graph is a photo of the whole wave at one instant. Crest to next crest is the wavelength.
x-axis = time: the graph shows one point (a microphone, your ear, a buoy) moving as the wave passes. Crest to next crest is the period.
Frequency and period
=
f = 1 ÷ T
frequency (Hz) = 1 ÷ period (s)
Rearranged: T = 1 ÷ f⏱
Seconds first
T must be in seconds. Change minutes to seconds (× 60) before you work out the period or the frequency.
A frequency of 5 Hz means 5 waves pass a point every second, so each one takes 1 ÷ 5 = 0.2 s. A high frequency means a short period.
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How to write it in the exam
The sentence frame for "describe a transverse (or longitudinal) wave"
The oscillations are
perpendicular (transverse) or parallel (longitudinal)
to the direction of energy transfer.
what moveswhich waycompared with what
Exam-style question · 6 marks
(a) Use diagrams to describe the difference between transverse and longitudinal waves. [4] (b) Give one example of each. [2]
Transverse: oscillation at right angles
Longitudinal: oscillation along the direction of travel
- Diagram of a transverse wave with the oscillation drawn at right angles to the direction of travel (1)
- Transverse: the oscillations are perpendicular to the direction of energy transfer (1)
- Diagram of a longitudinal wave showing compressions and rarefactions, oscillation drawn along the direction of travel (1)
- Longitudinal: the oscillations are parallel to the direction of energy transfer (1)
- (b) Transverse: light, any electromagnetic wave, or water ripples (1) · Longitudinal: sound or ultrasound (1)
Where marks go missing: unlabelled arrows. Write oscillation on one and direction of energy transfer on the other.
Worked example · amplitude from a graph · 1 mark
A graph shows a wave going from +2.0 cm to −2.0 cm. A student says the amplitude is 4.0 cm. Explain why they're wrong.
Rest position to the peak: 2.0 cm
- 4.0 cm is crest to trough. Amplitude is measured from the rest position to a crest.
- Amplitude = 2.0 cm
Worked example · frequency from a time graph · 2 marks
On a displacement–time graph, one crest is at 0 s and the next is at 0.20 s. Calculate the frequency.
The x-axis is time, so peak to peak is the period
- The x-axis is time, so crest to crest is the period: T = 0.20 s
- f = 1 ÷ T = 1 ÷ 0.20 = 5 Hz
Worked example · the minutes trap · 2 marks
A buoy bobs up and down 10 times in 2 minutes. Calculate the period of the buoy.
- 2 minutes = 2 × 60 = 120 s
- T = 120 ÷ 10 = 12 s
Then the frequency: f = 1 ÷ 12 = 0.083 Hz. Using 2 minutes instead of 120 s gives T = 0.2, which is wrong.
📘 Now do it in your workbook
Page 3: draw and label a transverse and a longitudinal wave, sort six waves into T or L, then the 6-mark exam question. Page 4: label amplitude, wavelength and period on three graphs and calculate a frequency.
AQA GCSE Physics Workbook · WAVES · pages 3 and 4The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 3 and 4 of the workbook have the diagrams to label, the graphs to read and the 6-mark exam question, with full answers at the back.
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