The shape of the lesson
By the end of the lesson, learners can
- Watch the air carry the sound: See the loudspeaker push the air into compressions and rarefactions, and notice that each particle only vibrates on the spot.
- Along the travel, not across it: In a longitudinal wave the vibration lies along the direction of travel, the opposite of a transverse wave.
- Longitudinal: particles vibrate parallel to the direction the wave travels.
- Compression and rarefaction: regions of higher and lower pressure as particles bunch and spread.
- Needs a medium: sound cannot travel through a vacuum.
One picture every idea returns to
Sound is a longitudinal wave: the particles of the medium vibrate back and forth along the direction the wave travels, making compressions where they bunch together and rarefactions where they spread apart. Sound needs a medium, so it cannot travel through a vacuum.
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, Squeeze and stretch.. A loudspeaker does not throw air across the room. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Watch the air carry the sound | See the loudspeaker push the air into compressions and rarefactions, and notice that each particle only vibrates on the spot. Project tab 1 of the lab (Longitudinal wave) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Along the travel, not across it | In a longitudinal wave the vibration lies along the direction of travel, the opposite of a transverse wave. | 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 Along the travel, not across it, 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 |
|---|---|
| Sound is longitudinal, not transverse. | The particles move back and forth along the wave direction, they do not move across it, and they do not travel along with the wave. Because there are no particles to vibrate, sound cannot cross the vacuum of space. |
Sound as a longitudinal wave: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Sound is described as a longitudinal wave. Explain what this means in terms of the motion of the particles.
As a sound wave passes through air, regions of compression and rarefaction form. State what is meant by each.
An electric bell is ringing inside a sealed glass jar. As a pump slowly removes the air from the jar, the sound gets quieter and quieter until it can no longer be heard, even though the bell is still seen striking. Explain this observation.
A student says that when someone speaks across a room, the air travels from their mouth to the listener's ear. Explain why this is not correct.
Waves can be longitudinal or transverse.
(a) State how the particle vibration in a transverse wave differs from that in a longitudinal wave. [1] (b) Give one example of a transverse wave. [1]In many science fiction films, an explosion in outer space is shown with a loud bang. Explain why, in reality, no sound would be heard.
Answer key and marking notes
- The particles vibrate back and forth about fixed positions. ✓
- Their vibration is parallel to (along) the direction in which the wave travels. ✓
- A compression is a region where the particles are pushed together, so the pressure is higher than normal. ✓
- A rarefaction is a region where the particles are spread apart, so the pressure is lower than normal. ✓
- Sound needs particles of a medium to travel through. ✓
- As the air is removed there are fewer particles to carry the sound, and in a vacuum there are none, so no sound can travel out of the jar. ✓
- Each air particle only vibrates back and forth about its own fixed position. ✓
- What travels across the room is the wave and the energy it carries, passed from particle to particle, not the air itself. ✓
(a) In a transverse wave the particles vibrate at right angles (perpendicular) to the direction of travel, rather than parallel to it. ✓
(b) Any electromagnetic wave, for example light (or water ripples, or a wave on a rope). ✓
- Outer space is very nearly a vacuum, with almost no particles. ✓
- Sound is a longitudinal wave that needs particles of a medium to travel, so with no medium the sound cannot travel and nothing is heard. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Sound travels as a longitudinal wave. This means it consists of:
- Acompressions and rarefactions along the direction of travel
- Bcrests and troughs at right angles to the direction of travel
- Ca steady stream of particles moving forward
- Dan electromagnetic wave that needs no medium
Why can sound not travel through a vacuum?
- AThere are no particles to vibrate and pass the energy on.
- BSound travels too quickly in a vacuum.
- CA vacuum absorbs all sound energy.
- DSound is an electromagnetic wave.
Quiz key
Sound is made of compressions (particles pushed together) and rarefactions (particles spread apart) along the direction of travel. Crests and troughs (B) belong to transverse waves.
Sound needs a medium because it is carried by vibrating particles; a vacuum has no particles, so the wave cannot travel. Electromagnetic waves (D) can cross a vacuum, but sound cannot.
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.