Wave properties and behaviour

What you'll learn

How waves transfer energy and information without transferring matter.
The key wave quantities: frequency, wavelength, amplitude, period, wave velocity and wavefront.
How to use the two wave-speed equations you must recall.
How waves behave at boundaries: reflection, refraction, transmission and absorption.

Waves transfer energy, not matter

A wave is a travelling disturbance that transfers energy from one place to another.

Waves can also transfer information. For example, sound waves carry speech, light waves carry images, and radio waves carry phone signals.

Key Idea

What moves in a wave?

Waves transfer energy and information without transferring matter overall. The particles of the material usually vibrate around fixed positions; they do not travel along with the wave.

Evidence from water waves

If a cork floats on water and a ripple passes, the cork moves up and down, but it does not travel across the water at the same speed as the ripple. The wave pattern moves across the surface; the water mainly oscillates.

Evidence from sound waves

Sound travels through air, but the air itself is not blown from the source to your ear. Air particles vibrate backwards and forwards around their positions, passing the disturbance onwards.

Example

Using evidence that matter is not transferred

A student watches a leaf floating on a pond as ripples pass from left to right. The leaf bobs up and down but stays in roughly the same place. Explain what this shows.

The ripple pattern moves from left to right, so energy is being transferred across the water surface.
The leaf is carried by nearby water particles, so its bobbing shows the water particles are oscillating.
Because the leaf does not move left to right with the ripple, the water itself is not travelling across the pond with the wave.

Describing waves

A vibration or oscillation is a repeated movement about a central position.

For waves, you need to know these terms:

Amplitude is the maximum displacement from the rest position. Bigger amplitude usually means more energy is being transferred.
Wavelength, symbol $λ\lambda$ , is the distance from one point on a wave to the same point on the next wave, such as crest to crest.
Frequency, symbol $f$ , is the number of complete waves passing a point each second. It is measured in hertz (Hz).
Period, symbol $T$ , is the time taken for one complete wave or oscillation. It is measured in seconds (s).
Wave velocity or wave speed, symbol $v$ , is how fast the wave travels, measured in metres per second (m/s).
A wavefront is a line joining points on a wave that are at the same stage of the oscillation, such as a line of crests in a ripple tank.

The diagram shows the main labels for transverse and longitudinal waves.

Diagram comparing transverse and longitudinal waves with amplitude, wavelength, crests, troughs, compressions and rarefactions labelled

Common Mistake

Amplitude is not wavelength

Amplitude is measured from the rest position to a crest or trough. Wavelength is measured along the direction the wave travels, from one matching point to the next.

Example

Finding frequency from oscillations

A floating ball completes 15 up-and-down oscillations in 5 seconds. Find the frequency.

Frequency means the number of complete oscillations per second.
Divide the number of oscillations by the time: $\div 5$ .
The frequency is $3\ \text{Hz}$ .

Transverse and longitudinal waves

Waves are classified by comparing the direction of vibration with the direction the wave travels.

In a transverse wave, the vibrations are perpendicular to the direction of energy transfer. “Perpendicular” means at 90 degrees.

Examples include:

electromagnetic waves, such as light, microwaves and radio waves
seismic S-waves
water surface waves, which are treated as transverse at GCSE

In a longitudinal wave, the vibrations are parallel to the direction of energy transfer. Longitudinal waves have compressions, where particles are closer together, and rarefactions, where particles are further apart.

Examples include:

sound waves in air
seismic P-waves

Key Idea

Direction test

For a transverse wave, vibration is perpendicular to travel. For a longitudinal wave, vibration is parallel to travel.

Example

Classifying a wave type

A wave travels horizontally along a spring. The coils vibrate backwards and forwards horizontally. Decide whether the wave is transverse or longitudinal.

The wave travels horizontally.
The coils also vibrate horizontally, so the vibration is parallel to the direction of travel.
A wave with parallel vibration and travel directions is longitudinal.

Wave speed equations

For Edexcel GCSE Physics, you must be able to recall and use both wave-speed equations:

\times \lambda

where:

$v$ is wave speed in metres per second (m/s)
$f$ is frequency in hertz (Hz)
$λ\lambda$ is wavelength in metres (m)

You must also recall:

\frac{x}{t}

where:

$x$ is distance travelled in metres (m)
$t$ is time taken in seconds (s)

Tip

Choosing the equation

Use $\times \lambda$ when you know frequency and wavelength. Use $\frac{x}{t}$ when you know distance travelled and time taken.

Example

Calculating wave speed from frequency and wavelength

A water wave has frequency $Hz8\ \text{Hz}$ and wavelength $m0.25\ \text{m}$ . Calculate its wave speed.

Choose the equation using frequency and wavelength: $\times \lambda$ .
Substitute the values with units: $8\ \text{Hz} \times 0.25\ \text{m}$ .
Calculate the answer: $2.0\ \text{m/s}$ .

Measuring wave velocity

Measuring the speed of sound in air

A good method uses two microphones connected to an oscilloscope or data logger.

Place the microphones a measured distance apart, such as 1.5 m.
Make a sharp sound, such as a clap, near the first microphone.
Measure the time delay between the sound reaching the first and second microphones.
Use $\frac{x}{t}$ .

This is more accurate than using a stopwatch, because sound travels very quickly in air.

Measuring the speed of ripples on water

You can use a ripple tank.

A vibrating dipper makes regular ripples on the water surface.
The frequency is set by the signal generator.
Measure the wavelength by measuring across several wave gaps, then dividing by the number of wavelengths.
Use $\times \lambda$ .

Example

Measuring ripple speed

In a ripple tank, the distance from the first crest to the sixth crest is $m0.20\ \text{m}$ . The frequency is $Hz12\ \text{Hz}$ . Calculate the ripple speed.

From the first crest to the sixth crest there are 5 wavelength gaps.
Find one wavelength: $m\lambda = 0.20\ \text{m} \div 5 = 0.040\ \text{m}$ .
Use the wave equation: $\times \lambda = 12\ \text{Hz} \times 0.040\ \text{m}$ .
The ripple speed is $0.48\ \text{m/s}$ .

Using echoes to find distance or depth

If you study Separate Physics at Higher Tier, you also need to calculate depth or distance from time and wave velocity.

An echo happens when a wave reflects from a boundary and returns to the detector. For sonar or ultrasound echoes, the measured time is often the time for the wave to go to the object and back again.

Common Mistake

Echo time is a round trip

If a pulse goes to a wall, seabed or object and returns, the total distance travelled by the wave is twice the distance to the object. Divide by 2 to find the one-way distance.

Example

Calculating depth from an echo

A sonar pulse travels through water at $m/s1500\ \text{m/s}$ . The echo returns after $s0.080\ \text{s}$ . Calculate the depth of the water.

Calculate the total distance travelled by the sound: $\times t = 1500\ \text{m/s} \times 0.080\ \text{s}$ .
This gives $120\ \text{m}$ for the journey down and back.
The depth is half the total distance: $m120\ \text{m} \div 2 = 60\ \text{m}$ .

Waves at material boundaries

A material interface is the boundary between two different materials, such as air and glass, air and water, or water and a wall.

When waves meet a boundary, several things can happen:

Reflection: the wave bounces back into the original material.
Transmission: the wave passes through into the next material.
Absorption: energy from the wave is transferred to the material, often heating it, so the wave amplitude decreases.
Refraction: the wave changes direction when it enters a different material.

The four named boundary effects are mainly Separate Physics content in this part of the Edexcel specification, but understanding them helps with all refraction questions.

Refraction

Refraction happens when a wave crosses a boundary and changes speed. If the wave enters the boundary at an angle, the change in speed causes a change in direction.

For Higher Tier, you should link refraction to a change in speed as well as direction. A very important idea is that the frequency stays the same at a boundary because it is set by the source. If speed changes, wavelength changes too, because $\times \lambda$ .

$Refraction diagram showing a wave slowing down at a boundary, bending towards the normal, and having a shorter wavelength$

Common Mistake

Frequency does not change at a boundary

When a wave enters a new material, its speed and wavelength may change, but its frequency stays the same.

Example

Predicting refraction from speed change

A light wave enters glass from air at an angle. It travels more slowly in glass than in air. Describe what happens.

The wave speed decreases when the light enters the glass.
Because the wave hits the boundary at an angle, the change in speed causes the wave to change direction.
When a wave slows down, it bends towards the normal, and its wavelength becomes shorter because the frequency stays the same.

Some substances absorb, transmit, refract or reflect waves differently depending on wavelength. This is Higher Tier content. For example, ordinary glass transmits most visible light but absorbs some ultraviolet waves. A material can therefore be transparent to one wavelength range but not another.

Exam technique

In the exam

For wave calculations, write down the equation first, then substitute values with units: $\times \lambda$ or $\frac{x}{t}$ .
For transverse and longitudinal questions, compare the vibration direction with the direction of wave travel.
For echo questions, check whether the time is for a one-way journey or a there-and-back journey.

Self review

Check yourself

What evidence shows that a water wave transfers energy but not water across the surface?
How are frequency, wavelength and wave speed linked?
What changes, and what stays the same, when a wave is refracted at a boundary?

Wave properties and behaviour

What you'll learn

How waves transfer energy and information without transferring matter.
The key wave quantities: frequency, wavelength, amplitude, period, wave velocity and wavefront.
How to use the two wave-speed equations you must recall.
How waves behave at boundaries: reflection, refraction, transmission and absorption.

Waves transfer energy, not matter

A wave is a travelling disturbance that transfers energy from one place to another.

Waves can also transfer information. For example, sound waves carry speech, light waves carry images, and radio waves carry phone signals.

Key Idea

What moves in a wave?

Waves transfer energy and information without transferring matter overall. The particles of the material usually vibrate around fixed positions; they do not travel along with the wave.

Evidence from water waves

Evidence from sound waves

Sound travels through air, but the air itself is not blown from the source to your ear. Air particles vibrate backwards and forwards around their positions, passing the disturbance onwards.

Example

Using evidence that matter is not transferred

A student watches a leaf floating on a pond as ripples pass from left to right. The leaf bobs up and down but stays in roughly the same place. Explain what this shows.

The ripple pattern moves from left to right, so energy is being transferred across the water surface.
The leaf is carried by nearby water particles, so its bobbing shows the water particles are oscillating.
Because the leaf does not move left to right with the ripple, the water itself is not travelling across the pond with the wave.

Describing waves

A vibration or oscillation is a repeated movement about a central position.

For waves, you need to know these terms:

Amplitude is the maximum displacement from the rest position. Bigger amplitude usually means more energy is being transferred.
Wavelength, symbol $λ\lambda$ , is the distance from one point on a wave to the same point on the next wave, such as crest to crest.
Frequency, symbol $f$ , is the number of complete waves passing a point each second. It is measured in hertz (Hz).
Period, symbol $T$ , is the time taken for one complete wave or oscillation. It is measured in seconds (s).
Wave velocity or wave speed, symbol $v$ , is how fast the wave travels, measured in metres per second (m/s).
A wavefront is a line joining points on a wave that are at the same stage of the oscillation, such as a line of crests in a ripple tank.

The diagram shows the main labels for transverse and longitudinal waves.

Diagram comparing transverse and longitudinal waves with amplitude, wavelength, crests, troughs, compressions and rarefactions labelled

Common Mistake

Amplitude is not wavelength

Amplitude is measured from the rest position to a crest or trough. Wavelength is measured along the direction the wave travels, from one matching point to the next.

Example

Finding frequency from oscillations

A floating ball completes 15 up-and-down oscillations in 5 seconds. Find the frequency.

Frequency means the number of complete oscillations per second.
Divide the number of oscillations by the time: $\div 5$ .
The frequency is $3\ \text{Hz}$ .

Transverse and longitudinal waves

Waves are classified by comparing the direction of vibration with the direction the wave travels.

In a transverse wave, the vibrations are perpendicular to the direction of energy transfer. “Perpendicular” means at 90 degrees.

Examples include:

electromagnetic waves, such as light, microwaves and radio waves
seismic S-waves
water surface waves, which are treated as transverse at GCSE

Examples include:

sound waves in air
seismic P-waves

Key Idea

Direction test

For a transverse wave, vibration is perpendicular to travel. For a longitudinal wave, vibration is parallel to travel.

Example

Classifying a wave type

A wave travels horizontally along a spring. The coils vibrate backwards and forwards horizontally. Decide whether the wave is transverse or longitudinal.

The wave travels horizontally.
The coils also vibrate horizontally, so the vibration is parallel to the direction of travel.
A wave with parallel vibration and travel directions is longitudinal.

Wave speed equations

For Edexcel GCSE Physics, you must be able to recall and use both wave-speed equations:

\times \lambda

where:

$v$ is wave speed in metres per second (m/s)
$f$ is frequency in hertz (Hz)
$λ\lambda$ is wavelength in metres (m)

You must also recall:

\frac{x}{t}

where:

$x$ is distance travelled in metres (m)
$t$ is time taken in seconds (s)

Tip

Choosing the equation

Use $\times \lambda$ when you know frequency and wavelength. Use $\frac{x}{t}$ when you know distance travelled and time taken.

Example

Calculating wave speed from frequency and wavelength

A water wave has frequency $Hz8\ \text{Hz}$ and wavelength $m0.25\ \text{m}$ . Calculate its wave speed.

Choose the equation using frequency and wavelength: $\times \lambda$ .
Substitute the values with units: $8\ \text{Hz} \times 0.25\ \text{m}$ .
Calculate the answer: $2.0\ \text{m/s}$ .

Measuring wave velocity

Measuring the speed of sound in air

A good method uses two microphones connected to an oscilloscope or data logger.

Place the microphones a measured distance apart, such as 1.5 m.
Make a sharp sound, such as a clap, near the first microphone.
Measure the time delay between the sound reaching the first and second microphones.
Use $\frac{x}{t}$ .

This is more accurate than using a stopwatch, because sound travels very quickly in air.

Measuring the speed of ripples on water

You can use a ripple tank.

A vibrating dipper makes regular ripples on the water surface.
The frequency is set by the signal generator.
Measure the wavelength by measuring across several wave gaps, then dividing by the number of wavelengths.
Use $\times \lambda$ .

Example

Measuring ripple speed

In a ripple tank, the distance from the first crest to the sixth crest is $m0.20\ \text{m}$ . The frequency is $Hz12\ \text{Hz}$ . Calculate the ripple speed.

From the first crest to the sixth crest there are 5 wavelength gaps.
Find one wavelength: $m\lambda = 0.20\ \text{m} \div 5 = 0.040\ \text{m}$ .
Use the wave equation: $\times \lambda = 12\ \text{Hz} \times 0.040\ \text{m}$ .
The ripple speed is $0.48\ \text{m/s}$ .

Using echoes to find distance or depth

If you study Separate Physics at Higher Tier, you also need to calculate depth or distance from time and wave velocity.

Common Mistake

Echo time is a round trip

If a pulse goes to a wall, seabed or object and returns, the total distance travelled by the wave is twice the distance to the object. Divide by 2 to find the one-way distance.

Example

Calculating depth from an echo

A sonar pulse travels through water at $m/s1500\ \text{m/s}$ . The echo returns after $s0.080\ \text{s}$ . Calculate the depth of the water.

Calculate the total distance travelled by the sound: $\times t = 1500\ \text{m/s} \times 0.080\ \text{s}$ .
This gives $120\ \text{m}$ for the journey down and back.
The depth is half the total distance: $m120\ \text{m} \div 2 = 60\ \text{m}$ .

Waves at material boundaries

A material interface is the boundary between two different materials, such as air and glass, air and water, or water and a wall.

When waves meet a boundary, several things can happen:

Reflection: the wave bounces back into the original material.
Transmission: the wave passes through into the next material.
Absorption: energy from the wave is transferred to the material, often heating it, so the wave amplitude decreases.
Refraction: the wave changes direction when it enters a different material.

The four named boundary effects are mainly Separate Physics content in this part of the Edexcel specification, but understanding them helps with all refraction questions.

Refraction

Refraction happens when a wave crosses a boundary and changes speed. If the wave enters the boundary at an angle, the change in speed causes a change in direction.

$Refraction diagram showing a wave slowing down at a boundary, bending towards the normal, and having a shorter wavelength$

Common Mistake

Frequency does not change at a boundary

When a wave enters a new material, its speed and wavelength may change, but its frequency stays the same.

Example

Predicting refraction from speed change

A light wave enters glass from air at an angle. It travels more slowly in glass than in air. Describe what happens.

The wave speed decreases when the light enters the glass.
Because the wave hits the boundary at an angle, the change in speed causes the wave to change direction.
When a wave slows down, it bends towards the normal, and its wavelength becomes shorter because the frequency stays the same.

Exam technique

In the exam

For wave calculations, write down the equation first, then substitute values with units: $\times \lambda$ or $\frac{x}{t}$ .
For transverse and longitudinal questions, compare the vibration direction with the direction of wave travel.
For echo questions, check whether the time is for a one-way journey or a there-and-back journey.

Self review

Check yourself

What evidence shows that a water wave transfers energy but not water across the surface?
How are frequency, wavelength and wave speed linked?
What changes, and what stays the same, when a wave is refracted at a boundary?

Wave properties and behaviour

What you'll learn

Waves transfer energy, not matter

Evidence from water waves

Evidence from sound waves

Describing waves

Transverse and longitudinal waves

Wave speed equations

Measuring wave velocity

Measuring the speed of sound in air

Measuring the speed of ripples on water

Using echoes to find distance or depth

Waves at material boundaries

Refraction

Recap questions

Wave properties and behaviour

What you'll learn

Waves transfer energy, not matter

Evidence from water waves

Evidence from sound waves

Describing waves

Transverse and longitudinal waves

Wave speed equations

Measuring wave velocity

Measuring the speed of sound in air

Measuring the speed of ripples on water

Using echoes to find distance or depth

Waves at material boundaries

Refraction

Recap questions

Wave properties and behaviour

What you'll learn

Waves transfer energy, not matter

Evidence from water waves

Evidence from sound waves

Describing waves

Transverse and longitudinal waves

Wave speed equations

Measuring wave velocity

Measuring the speed of sound in air

Measuring the speed of ripples on water

Using echoes to find distance or depth

Waves at material boundaries

Refraction

Recap questions

Wave properties and behaviour Revision Guide

Wave properties and behaviour

What you'll learn

Waves transfer energy, not matter

Evidence from water waves

Evidence from sound waves

Describing waves

Transverse and longitudinal waves

Wave speed equations

Measuring wave velocity

Measuring the speed of sound in air

Measuring the speed of ripples on water

Using echoes to find distance or depth

Waves at material boundaries

Refraction

Recap questions

Wave properties and behaviour Revision Guide