What is a wave?

Key idea: Build the wave model used across the shared G3 Physics and O-Level sequence.

SEC G3 Physics 2027

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Learning objectives

Describe wave generation by vibrating sources, ropes and springs
Describe ripple-tank waves using wavefronts
Explain that waves transfer energy
Explain that wave energy transfer does not transfer matter
Use amplitude, frequency and wavelength to describe wave motion
Define and use wave speed and period and interpret wave graphs
Recall and apply wave speed = frequency × wavelength
Compare transverse and longitudinal waves and give examples
Explain sound production by vibration and the need for a medium
Describe sound using compressions and rarefactions
Relate sound loudness to amplitude and pitch to frequency
Explain reflected-sound echoes and use them to measure distance
Explain ultrasound use in sonar and soft-tissue scanning

1. Definition

Wave motion is the transfer of energy from one place to another by a travelling disturbance, without a net transfer of matter.

2. Key Ideas

A wave needs a source of disturbance. For a continuous wave, that source oscillates repeatedly.
Particles in the medium oscillate about a fixed position; they do not travel with the wave.
Waves can be:
- mechanical (need a medium) e.g. water waves, sound
- electromagnetic (do not need a medium) e.g. light
Waves can be transverse or longitudinal.
Key quantities:
- amplitude A (m)
- wavelength λ (m)
- frequency f (Hz)
- period T (s)
- wave speed v (m s⁻¹)
Wave equation: v = fλ

You should be able to explain wave motion and wavefronts, describe energy transfer without net matter transfer, compare transverse and longitudinal waves, and use v = fλ.

3. Detailed Explanations

A. Wave motion: energy transfer without matter transfer

When a wave passes through a medium, each particle vibrates around its equilibrium position. The vibration is passed from particle to particle, so energy moves forward even though the particles do not.

B. Mechanical vs electromagnetic waves

Type	Needs a medium?	Example
Mechanical	Yes	Water waves, sound
Electromagnetic	No (can travel in vacuum)	Light, radio waves

C. Transverse vs longitudinal waves

Transverse wave: vibration is perpendicular to the direction the wave travels. Examples include waves on a stretched string and electromagnetic waves.

Longitudinal wave: vibration is parallel to the direction the wave travels (compressions and rarefactions).

Amplitude is measured from the equilibrium line; wavelength is measured between consecutive points in phase. In a longitudinal wave, one wavelength is the spacing between neighbouring compressions.

Feature	Transverse	Longitudinal
Direction of vibration	Perpendicular to wave travel	Parallel to wave travel
Named parts	Crests and troughs	Compressions and rarefactions
Examples	Waves on a stretched string, light	Sound in air, compression waves in a spring

D. Wavefront

A wavefront is an imaginary line (or surface) joining points that are in the same phase (e.g. crest-to-crest).

In a ripple tank:

a straight vibrating bar produces straight wavefronts
a point dipper produces circular wavefronts

Scroll diagram horizontally to read all labels.

A line source produces approximately straight wavefronts; a point source produces circular wavefronts. Each wavefront joins points in the same phase, and the arrows show energy propagation perpendicular to the wavefronts.

E. Reading a displacement–distance graph

At an instant in time, you can draw a graph of displacement against distance along the wave.

This is a spatial snapshot, so the horizontal axis gives position, not time. Read amplitude vertically from the equilibrium line and wavelength horizontally between consecutive points in the same phase. Do not use this graph to read the period unless the horizontal axis is time.

F. Definitions of wave quantities (with units)

Amplitude, A (m): maximum displacement from the equilibrium position.
Wavelength, λ (m): distance between two consecutive points in phase (e.g. crest to crest).
Frequency, f (Hz): number of complete oscillations or cycles passing a point each second (set by the source).
Period, T (s): time taken for one complete oscillation. T = 1/f
Wave speed, v (m s⁻¹): speed at which the disturbance travels. v = fλ

G. Video: transverse vs longitudinal

4. Common Mistakes

Saying particles “travel with the wave” (they oscillate about a fixed position).
Mixing up:
- amplitude (how big the vibration is)
- wavelength (distance between repeats)
Reading period from a displacement–distance graph; period belongs on a displacement–time graph.
Using inconsistent units in v = fλ (e.g. λ in cm but v in m s⁻¹).

5. Exam Tips

Always write the equation first, then substitute with units: v = fλ.
Convert units before substituting (cm → m, ms → s).
If asked “time for one wave”, use T = 1/f.
For wavefront questions, use the keyword same phase.

6. Worked Examples

Problem

A wave has speed

3.0 m s⁻¹ and frequency

2.0 Hz. Find its wavelength.

Study the worked solution

Choose and rearrange the equation
Method
Start with v = fλ and divide by frequency.
Reason
Wavelength is the unknown spatial repeat distance.
Working
λ = v/f
Substitute consistent units
Method
Divide metres per second by cycles per second.
Reason
The seconds cancel, leaving metres.
Working
λ = 3.0/2.0 = 1.5 m

Problem

A sound wave has frequency 250 Hz and wavelength 1.4 m. Find its speed.

Multiply the source rate by distance per cycle

Wave speed

Unit: m s^-1

Hints

Hint 1: recall the wave equation

Use v = fλ.

Hint 2: check units

Hertz and metres combine directly to metres per second.

View solution step by step

Apply the wave equation
Method
Multiply frequency by wavelength.
Reason
Each of 250 cycles per second advances one wavelength.
Working
v = (250)(1.4) = 350 m s⁻¹

Learner response

A source vibrates at 50 Hz. A student says its period is 50 s because period and frequency have the same numerical value. Locate the error.

Translate cycles per second into seconds per cycle

Period

Unit: s

View solution step by step

Use the reciprocal relationship
Method
Invert the frequency.
Reason
Frequency counts cycles per second, while period is seconds per cycle.
Working
T = 1/f = 1/50 = 0.020 s

Examination question

A water wave travels at

0.80 m s⁻¹ and has wavelength

4.0 cm. Find its frequency. [4 marks]

Show conversion, rearrangement and result

Write your four-mark solution

View solution step by step

Convert wavelength
1 mark
Method
Convert centimetres to metres.
Reason
The speed is expressed in metres per second.
Working
4.0 cm = 0.040 m
Calculate frequency
3 marks
Method
Rearrange v = fλ and divide speed by wavelength.
Reason
This gives cycles per second.
Working
f = v/λ = 0.80/0.040 = 20 Hz

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark conversion, equation, substitution and answer.

Mark points

Inverse-direction transfer

A source has period 0.25 s. Find its frequency and interpret the result in cycles per second.

Reverse the period calculation

Calculate and interpret frequency

Hints

Hint 1: use the reciprocal

f = 1/T.

Hint 2: interpret hertz

One hertz means one complete cycle per second.

View solution step by step

Calculate frequency
Method
Take the reciprocal of period.
Reason
Period is seconds per cycle; its reciprocal is cycles per second.
Working
f = 1/0.25 = 4.0 Hz
Interpret the value
Method
State four complete oscillations each second.
Reason
Hertz is the unit of cycles per second.
Working
4.0 Hz = 4.0 cycles per second.

Next step

Try Mind Stretcher 1 independently.

7. Mind Stretchers

Mind stretcher 1: Energy without matterExtension

People sometimes say “the wave carried water to the shore”. Explain why this is not a good description of wave motion.

Show Answer

In wave motion, the water particles mainly move up and down (or in small back-and-forth motions) around their equilibrium positions. The energy travels with the wave, but there is no net transfer of the water particles over large distances.

Mind stretcher 2: Changing frequencyExtension

In a ripple tank, the vibration frequency of the dipper is increased, but the water depth stays the same. What happens to wavelength? Explain briefly.

Show Answer

For the same water depth, wave speed is approximately constant. Since v = fλ, increasing f makes λ smaller.

8. Practice and next step

Sketch one displacement–distance graph and one displacement–time graph, then label amplitude, wavelength and period only where each can be read. Continue to the ripple-tank lesson to apply wavefront and wavelength ideas experimentally.

G3 Physics / O-Level practice check

After studying the sound and ultrasound lessons, use the General Properties of Waves check to find the wave idea to revisit first.

Practise

Practise: What is a wave?

A text-first General Waves assessment with explicit directions, particle spacing, graph quantities, echo times, speeds and units.

About 10 minutes

Practise

Questions are selected when you start. Use the feedback to decide what to practise next; this does not prove mastery.

Recent attempts

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No completed attempts are saved yet.

This check needs JavaScript to record answers. The reviewed questions are listed below for study.

Which statement correctly gives the phase relationship of points on one wavefront?

They reach their crests at different times along the line.
They are at the same stage of oscillation.
They have the same displacement but may be moving in opposite directions.
They are all the same distance from the edge of the tank.

Distinguish particle motion from energy transfer in a spring wave.

Coils oscillate locally while energy travels along the spring.
Each coil travels along the spring with the pulse and carries its energy.
The coils do not move; energy passes through a stationary spring.
Both the coils and the energy remain local, so nothing travels along the spring.

Which explanation correctly accounts for why period cannot be read as a distance?

Period is the maximum displacement and must be read vertically.
Period is a time for one cycle and must be read from a time axis.
Period is the distance between adjacent crests and must be read from a distance axis.
Period is a speed and therefore needs both distance and time axes.

Which option gives v for f = 25 Hz and λ = 0.12 m?

25.12 m s⁻¹
3.0 m s⁻¹
208 m s⁻¹
0.0048 m s⁻¹

A pulse travels to the right along a rope while a marked point on the rope moves up and down. How is the point's motion related to the pulse's travel?

The motion is perpendicular to the direction of travel.
The motion is parallel to the direction of travel.
The point travels to the right with the pulse.
The point remains stationary as the pulse passes.

A ripple-tank bar is to generate a regular wave train by completing 15 cycles in 3.0 s. Which driving instruction achieves this?

Hold the bar at a fixed height to generate five waves each second.
Move the bar up and down at 5.0 cycles per second.
Move the bar up and down at 45 cycles per second.
Move the bar up and down at 0.20 cycles per second.

Can sound travel through liquids and solids?

No. Sound can travel only through gases.
Yes, because sound can cross the empty gaps without particle interactions.
Only through liquids; particles in solids cannot vibrate.
Yes. Their particles can pass on mechanical disturbances.

In a repeating sound-wave compression diagram, which distance is one wavelength?

the width of one compression only
distance travelled by one air particle during a cycle
distance between the centres of adjacent compressions
distance from a compression to the adjacent rarefaction

Which wave quantity determines pitch?

echo time
frequency
amplitude
wave speed

What produces an echo?

absorption of sound by a soft surface
steady flow of air back from the surface
reflection of sound from a surface
refraction of sound as it enters a surface

Which statement correctly gives the measured data needed for ultrasound ranging?

pulse round-trip time and wave speed
pulse frequency and amplitude only
one-way time only, with no wave speed
wave speed and returned amplitude only

Compare an ultrasound pulse-echo sonar with an ultrasound medical scan. Which option distinguishes their purposes and states the principle used by both?

Sonar uses audible sound without reflection; medical scanning uses absorbed ultrasound.
Both methods use transmitted ultrasound only; neither needs echoes.
Both methods form tissue images, but sonar uses a different wave speed to create colour.
Both use reflected ultrasound; sonar ranges objects in water, while scanning maps tissue boundaries.

Practise

Practise after feedback: What is a wave?

A text-first General Waves assessment with explicit directions, particle spacing, graph quantities, echo times, speeds and units.

About 10 minutes

Practise

Questions are selected when you start. Use the feedback to decide what to practise next; this does not prove mastery.

Recent attempts

History is stored only in this browser.

No completed attempts are saved yet.

Check what I know

Check what I know: What is a wave?

A text-first General Waves assessment with explicit directions, particle spacing, graph quantities, echo times, speeds and units.

About 8 minutes

Check what I know

Answer 10 short questions. This starting check helps choose what to work on; it does not prove mastery.

Recent attempts

History is stored only in this browser.

No completed attempts are saved yet.

This check needs JavaScript to record answers. The reviewed questions are listed below for study.

What is a wavefront?

a line joining points at the same phase
a line joining points with the same displacement, even if they move in opposite directions
the path followed by one particle as the wave passes
the line along which the wave energy travels

In the ideal wave model, energy is transferred without net transfer of what?

wave speed
matter
energy
frequency

Which quantity is the horizontal time interval spanning one complete cycle on a displacement–time graph?

wavelength
amplitude
wave speed
period

A wave has f = 5.0 Hz and λ = 0.40 m. Find its speed.

2.0 m s⁻¹
12.5 m s⁻¹
0.080 m s⁻¹
5.4 m s⁻¹

How does vibration direction compare with travel in a transverse wave?

It is parallel to the direction of wave travel.
It is always in the direction of gravity.
It alternates between parallel and perpendicular every half-cycle.
It is perpendicular to the direction of wave travel.

In an ideal ripple-tank model, the water beside a dipper follows its vertical motion. Which driving motion produces a continuous wave train with amplitude 2.0 mm beside the dipper? Displacements are measured from the undisturbed water level.

Hold the dipper at a displacement of +2.0 mm.
Move the dipper repeatedly between +2.0 mm and −2.0 mm.
Move the dipper once between +2.0 mm and −2.0 mm, then stop.
Move the dipper repeatedly between +1.0 mm and −1.0 mm.

How does a tuning fork produce sound?

Its prongs move nearby air steadily away from the fork as a continuous flow.
Its vibrating prongs push and pull nearby particles, creating a travelling pressure disturbance.
Its prongs emit sound particles that travel to the listener.
Its prongs create sound in empty space without interacting with a medium.

What is a compression in a sound wave?

a region where particles are closer together and pressure is higher
a region where particles are farther apart and pressure is lower
a group of particles travelling together from source to listener
a region where particles stop vibrating and pressure is zero

For sound of a fixed frequency in the same medium, which wave quantity is associated with loudness?

wavelength alone
wave speed
amplitude
frequency

A ship's sonar sends an ultrasound pulse straight down. The echo from the seabed is received 0.80 s after the pulse is sent. Sound travels at 1500 m/s in seawater. Which statement is correct?

The seabed is 600 m deep; the pulse is ultrasound because its frequency is above the upper limit of human hearing.
The seabed is 1200 m deep; the pulse is ultrasound because its frequency is above the upper limit of human hearing.
The seabed is 600 m deep; the pulse is ultrasound because it travels faster than audible sound in seawater.
The seabed is 1875 m deep; the pulse is ultrasound because its frequency is above the upper limit of human hearing.

Check my progress

Check my progress: What is a wave?

A text-first General Waves assessment with explicit directions, particle spacing, graph quantities, echo times, speeds and units.

About 10 minutes

Check my progress

Answer 12 questions. If accepted, this result can contribute to your course progress.

Recent attempts

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Check again

Check again: What is a wave?

A text-first General Waves assessment with explicit directions, particle spacing, graph quantities, echo times, speeds and units.

About 10 minutes

Check again

Answer 11 questions. If accepted, this result can contribute to your course progress.

Recent attempts

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Review

Review: What is a wave?

A text-first General Waves assessment with explicit directions, particle spacing, graph quantities, echo times, speeds and units.

About 10 minutes

Review

Answer 12 questions. A scheduled review can contribute to your course progress only when it is due and the result is accepted.

Recent attempts

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Course and syllabus information

Course: SEC G3 Physics
Edition: SEC G3 Physics 2027

Continue where you stopped

Learning objectives

1. Definition

2. Key Ideas

3. Detailed Explanations

A. Wave motion: energy transfer without matter transfer

B. Mechanical vs electromagnetic waves

C. Transverse vs longitudinal waves

D. Wavefront

E. Reading a displacement–distance graph

F. Definitions of wave quantities (with units)

G. Video: transverse vs longitudinal

4. Common Mistakes

5. Exam Tips

6. Worked Examples

7. Mind Stretchers

Mind stretcher 1: Energy without matterExtension

Mind stretcher 2: Changing frequencyExtension

8. Practice and next step

G3 Physics / O-Level practice check

Practice complete

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Starting check complete

Progress check complete

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