Introduction to Waves
In 2004, the Indian Ocean tsunami travelled 5,000 km in under 2 hours, animals fled the coast minutes before it hit.
● Know
- That a wave transfers energy without transferring matter
- The key parts of the wave model: oscillation, medium and propagation
- Four examples of waves: water, sound, light and seismic
● Understand
- Why the water in a ripple does not travel with the wave
- That mechanical waves need a medium while electromagnetic waves do not
- How oscillations create the travelling disturbance we call a wave
● Can do
- Describe energy transfer by waves in everyday situations
- Identify the medium a wave travels through
- Distinguish between mechanical and electromagnetic waves conceptually
Drop a stone into a still pond and watch a cork floating nearby: the cork bobs up and down as each ripple passes, but it never drifts across the water to the far bank. A wave is a disturbance that transfers energy from one place to another without permanently displacing the medium. The water molecules oscillate around their equilibrium positions and return, the energy travels, the water does not.
This is a profound idea: waves carry energy and information, not matter. This is why you can hear sounds from distant sources without air molecules travelling from the source to your ear. It is also why light from distant stars reaches us through the vacuum of space.
Waves are everywhere in nature and technology. Sound waves carry communication. Light waves carry vision and photosynthesis. Water waves shape coastlines. Seismic waves reveal Earth interior. Radio waves connect our devices. Understanding waves is fundamental to understanding both the natural world and modern technology.
A Mexican wave in a stadium is a perfect human-scale analogy. Spectators briefly stand and sit in sequence, creating a visible wave that travels around the stadium. Each person only moves up and down (oscillates) but does not travel with the wave. The wave transfers energy and information (the signal to stand) around the stadium without anyone leaving their seat. If the stadium were empty, the wave could not propagate - it needs a medium (the crowd). This illustrates both that waves transfer energy without matter transport, and that mechanical waves require a medium.
Australian wave research: The Australian Coastal Oceanography group studies how ocean waves interact with Australia 25,760 km of coastline. Their research informs coastal engineering, surf forecasting, and climate adaptation. The Bureau of Meteorology wave models predict swell conditions for shipping, fishing, and surfing. Australian researchers also study seismic waves from earthquakes and mining explosions to understand the structure of the Australian continent crust and mantle.
Waves transport matter from the source to the receiver. This is false. In a water wave, a floating cork bobs up and down but does not travel with the wave. In a sound wave, air molecules oscillate around fixed positions but do not flow from speaker to ear. In light waves, photons do travel, but the electromagnetic field oscillates without requiring a material medium. The energy travels; the medium (if any) merely oscillates.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
All waves share fundamental properties that describe their size, speed, and energy.
Amplitude (A): The maximum displacement from the equilibrium position. Larger amplitude means more energy. For sound, amplitude corresponds to loudness. For light, amplitude corresponds to brightness (intensity).
Wavelength (λ): The distance between two consecutive identical points on the wave (e.g., crest to crest, or trough to trough). Measured in metres. Shorter wavelength means higher frequency for a given speed.
Frequency (f): The number of complete oscillations per second. Measured in hertz (Hz). Higher frequency means more oscillations per second. For sound, frequency corresponds to pitch. For light, frequency corresponds to colour.
Period (T): The time for one complete oscillation. T = 1/f.
Speed (v): How fast the wave disturbance travels. v = f × λ. Wave speed depends on the medium, not on frequency or amplitude.
A tuning fork vibrating at 440 Hz produces a sound wave with wavelength about 0.78 m in air (since v = 343 m/s, λ = v/f = 343/440 = 0.78 m). If you play the same note on a guitar, the string vibrates at 440 Hz but the sound wave in air still has the same wavelength because the frequency and speed are the same. However, the guitar note has a more complex waveform with multiple harmonics, giving it a different timbre. The fundamental frequency determines the pitch; the amplitude determines the loudness; the harmonic content determines the timbre.
Australian acoustic engineering: The Sydney Opera House is renowned for its acoustics, designed by acoustician Vilhelm Lothar Jordan. The concert hall geometry, materials, and surface treatments control how sound waves reflect, absorb, and diffuse. Australian acousticians use wave physics to design concert halls, recording studios, and noise barriers for highways. The Australian Research Council funds research into metamaterials that can manipulate sound waves in novel ways, potentially leading to better noise cancellation and medical imaging.
Wave speed depends on frequency. This is false for most waves in a given medium. In air, all sound waves travel at approximately 343 m/s regardless of frequency. In a vacuum, all light waves travel at 3 × 10^8 m/s regardless of colour. Wave speed is determined by the properties of the medium (elasticity, density, temperature), not by the wave characteristics. The exception is dispersion, where speed does depend on frequency in certain media (like prisms separating white light into colours), but this is a special case, not the general rule.
The wave equation v = fλ is one of the most important equations in physics. It connects the three fundamental wave properties.
If you know any two of speed, frequency, and wavelength, you can calculate the third. This equation applies to all waves: sound, light, water, seismic, and matter waves.
Key insight: When a wave changes medium, its frequency stays the same (determined by the source), but its speed changes (determined by the new medium). Since v = fλ and f is constant, the wavelength must change proportionally with speed.
For example, light entering water slows down (v decreases), so its wavelength decreases. But the frequency (colour) stays the same. Sound entering a denser medium speeds up, so its wavelength increases.
This principle is why refraction occurs: when waves enter a new medium at an angle, the change in speed causes the wavefront to bend.
A radio station broadcasts at 100 MHz (100 × 10^6 Hz). What is the wavelength?
v = fλ -> λ = v/f = (3 × 10^8 m/s) / (100 × 10^6 Hz) = 3 m.
This is why FM radio antennas are often about 3 metres long (or a fraction thereof) - they are tuned to resonate at this wavelength. If the same signal travelled through a coaxial cable where the speed is 2 × 10^8 m/s, the wavelength would be 2 m instead of 3 m. The frequency is still 100 MHz (set by the transmitter), but the wavelength adjusts to the new speed.
Australian radio astronomy: The Australian Square Kilometre Array Pathfinder (ASKAP) in Western Australia detects radio waves from distant galaxies with wavelengths from a few centimetres to metres. Radio astronomers use the wave equation to calculate the frequency of detected signals and identify their sources. The Square Kilometre Array (SKA), currently under construction in Australia and South Africa, will be the world largest radio telescope, detecting incredibly faint radio waves from the early universe. Understanding wave properties is essential for interpreting these cosmic signals.
Changing the amplitude of a wave changes its speed. This is false. Amplitude affects the energy carried by the wave but not its speed. A loud sound and a quiet sound travel at the same speed in air (343 m/s at 20C). A bright light and a dim light travel at the same speed in vacuum (3 × 10^8 m/s). Amplitude is independent of speed. However, in some nonlinear media, very large amplitudes can slightly affect speed, but this is a secondary effect not relevant at normal wave intensities.
Match each wave property to its definition.
Wrong: "Waves transfer matter as well as energy." No � waves transfer energy, not matter. The water in a ripple stays in the pond; the air in a sound wave stays near the speaker. Only the disturbance travels.
Right: Waves transfer energy, not matter. The particles in the medium vibrate around their rest positions while the disturbance moves forward. A floating leaf on a rippled pond bobs up and down but does not travel across the pond with the wave.
Wrong: "Waves only exist in water." No � water waves are just one type. Sound, light, seismic waves, radio waves and microwaves are all waves with different properties and applications.
Right: Waves exist in many forms beyond water. Sound waves travel through air and solids; light and radio waves travel through the vacuum of space; seismic waves travel through rock. Water waves are simply the most visible example we encounter in everyday life.
Wrong: "Sound and light are the same kind of wave." No � sound is a mechanical wave that needs a medium; light is an electromagnetic wave that can travel through a vacuum. They behave differently and travel at very different speeds.
Right: Sound and light are fundamentally different wave types. Sound is a mechanical wave that needs a medium to travel, it cannot travel through space. Light is an electromagnetic wave that requires no medium and travels at 300,000 km/s in a vacuum, nearly a million times faster than sound in air.
Waves in Australian Life
Surf culture: Australia's coastline is famous for its waves, and understanding how swells form and travel helps surfers predict the best breaks. Ocean swells are energy transferred through water from distant storms, the water particles themselves move in small circles, but the energy can travel thousands of kilometres.
Great Barrier Reef: Light penetration in water is critical for coral survival. As sunlight enters the ocean, water absorbs different colours at different depths, red light disappears first, while blue light penetrates deepest. This is why deep water looks blue and why corals live in relatively shallow, sunlit zones.
Aboriginal understanding of seismic signals: Aboriginal and Torres Strait Islander Peoples have long observed and interpreted natural signs from the land, including tremors and vibrations. Traditional knowledge systems recognise that the earth itself can carry signals over distance, reflecting an understanding of energy transfer through a medium that aligns with the modern wave model.
✍ Copy Into Your Books
▾What Is a Wave?
- A wave transfers energy without transferring matter
- The medium oscillates but does not travel with the wave
The Wave Model
- Oscillation: repeated back-and-forth movement
- Medium: material the wave travels through
- Propagation: how the wave spreads from source
Two Types of Waves
- Mechanical waves: need a medium (sound, water, seismic)
- Electromagnetic waves: can travel through vacuum (light)
Wave Detective
Explain the Energy Transfer
At the start of this lesson you were shown the leaf bobbing on a pond as ripples pass beneath it, without actually travelling across the water.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. 1. Explain what is meant by the statement "waves transfer energy without transferring matter." Use one example from the lesson to illustrate your answer. 4 MARKS
Q2. 2. Compare mechanical waves and electromagnetic waves. Include in your answer: (i) whether each needs a medium, and (ii) one example of each type. 4 MARKS
Q3. 3. Aboriginal and Torres Strait Islander Peoples have long observed natural signs from the land, including vibrations and tremors. Explain how this traditional knowledge connects to the scientific wave model of energy transfer through a medium. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Would you now explain the leaf-on-water observation differently?
- Can you apply the wave model to another example from your daily life?
Model answers (click to reveal)
Answers
▾MCQ 1
BA wave transfers energy from one place to another without transferring matter. The particles of the medium oscillate but do not travel with the wave.
MCQ 2
CIn a water wave, each water particle oscillates up and down (or in small circles) around a fixed position. The energy moves through the water, not the water itself.
MCQ 3
AA sound wave travelling through air is a mechanical wave because it requires a medium (air) to travel. Light, radio signals and X-rays are all electromagnetic waves.
MCQ 4
DThe Moon has no atmosphere (no medium), so sound waves cannot travel to the astronaut's ears. Light is an electromagnetic wave and does not need a medium, so it can travel through the vacuum of space.
MCQ 5
BThe claim is incorrect because it generalises from one type of wave (electromagnetic) to all waves. Mechanical waves such as sound require a medium and cannot travel through a vacuum. Only electromagnetic waves can do so.
Short Answer 1
Model answer: Waves transfer energy from one place to another without the matter itself moving the full distance. In a wave, particles of the medium oscillate around a fixed position and pass the disturbance to neighbouring particles. For example, when a stone is dropped into a pond, ripples spread outward but a floating leaf bobs up and down without travelling to the edge. This shows that energy has travelled across the pond, while the water (matter) has stayed in place.
Short Answer 2
Model answer: Mechanical waves require a medium to travel through, they cannot move through a vacuum. An example is a sound wave, which needs air, water or a solid material. Electromagnetic waves do not need a medium and can travel through a vacuum as well as through matter. An example is light from the Sun, which travels through the empty space between the Sun and Earth. This difference explains why we can see the Sun but cannot hear sounds from space.
Short Answer 3
Model answer: Aboriginal and Torres Strait Islander Peoples' observation of vibrations and tremors in the land reflects an understanding that energy can travel through a medium (the Earth) without the medium itself moving to a new location. This aligns with the scientific wave model, where seismic waves propagate through rock by making particles oscillate, transferring energy across distance. Traditional knowledge systems recognised that signals from the earth could indicate events happening far away, demonstrating a practical grasp of energy transfer through a medium that parallels the modern concept of wave propagation.