A wave can travel across a rope, through air, or through empty space. What actually moves is not the matter itself, but a disturbance carrying energy from one place to another.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
A cork floating on water bobs up and down as ripples pass. The ripple moves across the pond, but the cork mostly stays in the same area. If the cork is not travelling with the ripple, what is the wave actually moving?
Type your initial explanation below. You will revisit it at the end.
Write your initial explanation in your book. You will revisit it at the end.
Keep this idea in mind through the lesson: what moves, and what only oscillates?
Wrong: Work and energy are completely different concepts.
Right: Work is the transfer of energy; they share the same unit (joules) and are fundamentally linked.
📚 Core Content
A wave is a disturbance that transfers energy from one place to another. The medium, if there is one, usually oscillates around an equilibrium position instead of travelling along with the wave.
That distinction is the foundation of the whole module. In a ripple tank, the pattern travels across the surface, but each water particle mostly moves up and down or in small circular paths. In a sound wave, the air does not rush across the room from the speaker to your ear. Instead, air particles oscillate back and forth while the disturbance moves outward. This is fundamentally different from projectile motion, where the object itself travels from launch point to landing point.
This is why waves are best thought of as travelling patterns of energy transfer. The pattern moves. The particles of the medium respond locally. If you tie a ribbon to a stretched rope and send a pulse along it, the ribbon flicks up and down but does not travel with the pulse. The energy of your hand motion has been passed from particle to particle, but the rope material stays where it is.
The disturbance progresses across the surface, while markers in the medium oscillate about their positions.
Some waves need matter to carry the disturbance. Others can travel through a vacuum.
Mechanical waves require a medium. The particles of that medium oscillate and pass the disturbance to neighbouring particles. Sound waves in air, water waves, seismic waves, and a pulse on a rope are all mechanical waves. Without the medium, there is nothing to displace, so the wave cannot exist. This is why there is no sound in space — despite what movies suggest — and why a bell ringing inside an evacuated jar becomes inaudible as the air is pumped out.
Electromagnetic waves do not need a material medium. Light, radio waves, X-rays, and microwaves can travel through empty space. This is why sunlight can reach Earth from the Sun across the vacuum of space. Electromagnetic waves consist of oscillating electric and magnetic fields that regenerate each other as they propagate. At this level, it is enough to know that they are self-propagating and do not rely on matter particles to carry them forward.
Wave type is classified by how the particles of the medium move relative to the direction the wave travels.
In a transverse wave, the disturbance is perpendicular to the direction of travel. A rope flicked up and down is the standard example. The rope segments move vertically while the pulse moves horizontally. Water surface ripples and all electromagnetic waves are transverse. In a longitudinal wave, the disturbance is parallel to the direction of travel. Sound is the classic example: compressions and rarefactions travel forward while air particles oscillate back and forth along the same line.
This distinction matters because it determines how we draw wave diagrams and what properties we emphasise. Transverse waves have clear crests and troughs. Longitudinal waves have compressions and rarefactions. Both types, however, obey the same fundamental wave equation $v = f\lambda$.
Transverse: particle motion is perpendicular to propagation. Longitudinal: particle motion is parallel to propagation.
Later calculations only work if the vocabulary is clean from the start.
Even in a mostly conceptual lesson, we need the language that all later graph work and equations depend on. These terms describe what a wave is doing, not what we guess it is doing. Precision in vocabulary separates Band 4 from Band 6 responses. For example, amplitude is not "the height of the wave" — it is the maximum displacement from equilibrium. Wavelength is not "the length of one bump" — it is the spatial period, the distance over which the wave repeats.
Displacement changes continuously as the wave passes a point. Amplitude is the maximum value of that displacement. Period and frequency are time-based properties that belong to the source; wavelength and wave speed are space-based properties that describe how the disturbance is distributed. Keeping these categories separate helps prevent the common error of mixing up period and wavelength on graphs.
How far a particle is from equilibrium at a given instant.
The maximum displacement from equilibrium.
The distance between repeating points in phase, such as crest to crest.
The time for one complete oscillation.
The number of oscillations each second, measured in hertz.
How fast the disturbance travels through the medium or field.
A wave is one of nature's most efficient ways to move energy over long distances without moving the source material.
Consider ocean swell generated by a storm in the Southern Ocean. The energy from the storm can travel thousands of kilometres to break on Australian beaches, yet the water molecules in the Southern Ocean do not travel with it. Each molecule passes energy to its neighbour through a small local displacement. The energy moves; the matter stays. This is fundamentally different from convection currents, where the fluid itself moves and carries heat with it.
The amount of energy carried by a wave depends on both its amplitude and its frequency. Higher amplitude means more energy per oscillation. Higher frequency means more oscillations per second. Together, these determine the power delivered by the wave. This principle explains why a high-frequency ultrasound wave can deliver enough energy to image a foetus, and why a tsunami — with enormous amplitude and wavelength — can carry devastating energy across an ocean basin.
| Feature | Wave Motion | Particle Motion |
|---|---|---|
| Energy | Transferred along the direction of propagation | Not transferred over large distances |
| Matter | No bulk transport of medium | Local oscillation about equilibrium |
| Speed | Determined by medium properties | Varies with position in the wave |
✏️ Worked Examples
Scenario: For each case, identify whether the wave is mechanical or electromagnetic, and whether it is transverse or longitudinal.
A sound wave tried to move through outer space? Nothing would be there to oscillate, so the disturbance could not propagate. That single idea separates mechanical from electromagnetic waves.
Scenario: A student ties a small ribbon to the middle of a stretched horizontal rope and creates a single transverse pulse by flicking the rope upward at one end. Describe the motion of the ribbon as the pulse passes, and explain whether the ribbon travels horizontally with the pulse.
The student sent a longitudinal compression pulse along a slinky instead? The ribbon (if attached to one coil) would oscillate horizontally back and forth parallel to the slinky, again staying at roughly the same overall position while the disturbance moves through.
Visual Break
🏃 Activities
For each example below, decide:
A student says, "The cork moved upward, so the wave must be carrying the cork upward across the pond." Write a short response correcting this statement using the ideas of oscillation, disturbance, and energy transfer.
Draw a transverse wave and label equilibrium position, crest, trough, amplitude, and wavelength. Under your diagram, write one sentence that distinguishes amplitude from wavelength.
A single pulse is sent along a stretched rope. At the same time, a ball is thrown horizontally across the room.
Earlier you were asked: If the cork is not travelling with the ripple, what is the wave actually moving?
The full answer: the wave is moving a disturbance that transfers energy. The cork and nearby water particles oscillate about their positions, but the medium itself is not carried across the pond with the ripple. This is the essential difference between wave motion and object motion.
Now revise your first answer. What did you understand early, and what needed correcting?
Annotate your first answer in your book with what you now understand more clearly.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
✅ Check Your Understanding
1. Which statement best defines a wave?
2. Which wave can travel through a vacuum?
3. In a transverse wave, the particles of the medium move:
4. A sound wave in air is best described as:
5. A student says, "The water in the entire pond moves from the source to the edge because the wave moves outward." What is the best correction?
6. Explain the difference between a mechanical wave and an electromagnetic wave. Give one example of each. 3 MARKS
7. A pulse travels along a rope from left to right. Describe the motion of one marked particle on the rope as the pulse passes, and explain why that particle does not travel with the pulse to the far end. 3 MARKS
8. A student claims that all waves must be transverse because "waves go up and down." Evaluate this statement by referring to both sound waves and light waves. 4 MARKS
Ocean swell: mechanical; treated at this level as transverse surface motion; water particles oscillate locally.
FM radio signal: electromagnetic; no medium required.
Seismic P-wave: mechanical and longitudinal; rock particles oscillate parallel to propagation.
Guitar string: mechanical and transverse; string segments move perpendicular to the pulse.
Light from a torch: electromagnetic; no material medium required.
Similarity: Both the pulse and the ball transfer energy from one place to another. The pulse transfers energy along the rope; the ball transfers kinetic energy through space.
Difference: The ball itself travels from thrower to target, but the rope material only oscillates locally about its rest position. The pulse moves through the rope without the rope material travelling with it.
Why incorrect: It is incorrect to say rope particles travel with the pulse because a wave transfers energy, not matter. Each particle receives a small displacement and passes the energy to the next particle, then returns to equilibrium.
1. B — a wave is a disturbance that transfers energy.
2. D — visible light is electromagnetic and can travel through vacuum.
3. A — transverse means perpendicular particle motion.
4. C — sound in air is mechanical and longitudinal.
5. B — the disturbance moves, but the medium mainly oscillates locally.
Q6 (3 marks): A mechanical wave requires a medium whose particles oscillate and pass on the disturbance. An electromagnetic wave does not require a material medium and can travel through vacuum. Example mechanical wave: sound in air or a rope pulse. Example electromagnetic wave: visible light or a radio wave.
Q7 (3 marks): One marked particle on the rope moves up and then back down as the pulse passes. It oscillates about its equilibrium position. The pulse transfers energy along the rope, but the rope particle does not travel to the far end because the disturbance propagates while the medium responds locally.
Q8 (4 marks): The statement is incorrect. Not all waves are transverse. Sound waves in air are longitudinal because the air particles oscillate back and forth parallel to the direction the wave travels, creating compressions and rarefactions. Light waves are electromagnetic and do not require a medium. At this level they are distinguished from sound because they can travel through vacuum. So "waves go up and down" is only a useful picture for some transverse wave examples, not for all waves.
Climb platforms using your knowledge of wave motion and types of waves. Pool: lesson 1.
Tick when you have finished the activities and checked the answers.