The shape of the lesson
By the end of the lesson, learners can
- Two ways to wave: Switch between transverse and longitudinal and watch how the particles move relative to the energy travel.
- Side by side: The same ideas, two geometries.
- Why polarisation is the test: The two types behave differently when passed through a polariser.
- Transverse: oscillation perpendicular to travel.
- Longitudinal: oscillation along travel.
- Test: only transverse waves polarise.
One picture every idea returns to
In a transverse wave the oscillations are perpendicular to the direction of energy travel (light, waves on strings). In a longitudinal wave the oscillations are parallel to it, producing compressions and rarefactions (sound). Only transverse waves can be polarised.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Across or along.. Every mechanical wave moves its medium in one of two ways: side to side across the direction of travel, or back and forth along it. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Two ways to wave | Switch between transverse and longitudinal and watch how the particles move relative to the energy travel. Project tab 2 of the lab (Transverse and longitudinal) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Side by side | The same ideas, two geometries. | Pairs |
| 21 to 30 min | Teach and check: Why polarisation is the test | The two types behave differently when passed through a polariser. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why polarisation is the test, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not confuse the graph shape with the motion. | A displacement-distance graph of a longitudinal wave often looks like a transverse sine curve, but the real particle motion is back and forth along the travel direction. The distance between adjacent compressions is one whole wavelength, just as between adjacent crests. |
Transverse and longitudinal waves: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
State the difference between a transverse and a longitudinal wave, giving one example of each.
Explain why light can be polarised but sound cannot.
For a longitudinal sound wave, state what is meant by a compression and by a rarefaction.
Classify each as transverse or longitudinal: a water surface ripple, a sound wave, a wave on a string, a seismic P-wave (a push-pull wave).
A slinky is pushed back and forth along its length, sending a wave down it. State and justify the type of wave.
Explain why a displacement-distance graph of a longitudinal wave can look like a transverse sine curve.
Answer key and marking notes
- Transverse: oscillations perpendicular to the energy travel, for example light ✓
- Longitudinal: oscillations along the energy travel, for example sound ✓
- Light is transverse, so its oscillations lie in planes perpendicular to travel and one plane can be selected ✓
- Sound is longitudinal, with oscillation along the travel direction, so there is no plane to select ✓
- A compression is a region where the particles are pushed close together (high pressure) ✓
- A rarefaction is a region where the particles are spread apart (low pressure) ✓
- Transverse: water ripple and wave on a string ✓
- Longitudinal: sound wave and seismic P-wave ✓
- It is longitudinal ✓
- The coils oscillate back and forth along the direction the wave travels, forming compressions and rarefactions ✓
- The graph plots particle displacement (taken as positive one way, negative the other) against position ✓
- Plotted this way the back-and-forth displacements trace a sine curve, even though the real motion is along the travel direction ✓
The lesson's own checks as a quiz
The quick-check and exam-style questions from the student lesson, printable with the key.
In a transverse wave, the oscillations are:
- Aperpendicular to the direction of energy travel
- Bparallel to the direction of energy travel
- Cat 45° to the travel
- Dabsent
In a longitudinal wave, the particles oscillate:
- Aparallel to the direction of energy travel
- Bperpendicular to the travel
- Cin circles
- Dnot at all
Which of these is a longitudinal wave?
- Asound in air
- Blight
- Ca wave on a string
- Da water surface ripple
Regions of a longitudinal wave where the particles are pushed closest together are called:
- Acompressions
- Brarefactions
- Ccrests
- Dnodes
Which type of wave can be polarised?
- Atransverse waves only
- Blongitudinal waves only
- Cboth transverse and longitudinal
- Dneither
A sound wave and a light wave have the same frequency. Which statement is correct?
- Athe sound is longitudinal and the light is transverse
- Bboth are longitudinal
- Cboth are transverse
- Dthe sound is transverse and the light is longitudinal
The distance between two adjacent compressions in a longitudinal wave is equal to:
- Aone wavelength
- Bhalf a wavelength
- Ca quarter of a wavelength
- Dtwo wavelengths
Quiz key
Transverse: the medium moves at right angles to the direction the energy travels.
Longitudinal: the oscillation is along the direction of travel, giving compressions and rarefactions.
Sound is a longitudinal wave; the others listed are transverse.
Compressions are high-pressure regions where particles bunch up; rarefactions are the low-pressure regions between them.
Only transverse waves can be polarised, because only they oscillate in planes perpendicular to the travel direction.
Sound is always longitudinal; light (an electromagnetic wave) is transverse.
Adjacent compressions are one complete cycle apart, which is one wavelength.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.