GCSE Physics revision

Transverse and longitudinal waves

Waves in air, fluids and solids

AQA 4.6.1.1

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Describe the difference between longitudinal and transverse waves.

MOST 🎯🎯

Apply the scientific explanation of transverse and longitudinal waves to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving transverse and longitudinal waves and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is Refraction?

Refraction is when waves change direction as they pass from one medium to another, such as from air into glass.
Waves travel at different speeds in different mediums because different materials have different densities.
For electromagnetic waves like light, the higher the density of the material, the slower the wave travels through it.

1. What is Refraction?AQA

Notes

Refraction is when waves change direction as they pass from one medium to another, such as from air into glass.
Waves travel at different speeds in different mediums because different materials have different densities.
For electromagnetic waves like light, the higher the density of the material, the slower the wave travels through it.

How Waves Bend at a Boundary

If a wave travels perpendicular to the boundary between two mediums, it continues straight on without changing direction.
If a wave hits the boundary at an angle, it refracts — meaning its direction changes.
When light passes into a more dense medium (e.g. air into glass), it slows down and bends towards the normal.
When light passes into a less dense medium (e.g. glass into air), it speeds up and bends away from the normal.

2. How Waves Bend at a BoundaryAQA

Notes

If a wave travels perpendicular to the boundary between two mediums, it continues straight on without changing direction.
If a wave hits the boundary at an angle, it refracts — meaning its direction changes.
When light passes into a more dense medium (e.g. air into glass), it slows down and bends towards the normal.
When light passes into a less dense medium (e.g. glass into air), it speeds up and bends away from the normal.

Key Terms in Refraction

The normal is a dashed line drawn perpendicular to the surface at the point where the light ray meets the boundary.
The incident ray is the incoming light ray approaching the boundary.
The refracted ray is the ray that has changed direction inside the new medium.
The emergent ray is the ray that exits the second medium back into the original medium.
The angle of incidence and angle of refraction are both measured from the normal, not from the surface.

3. Key Terms in RefractionAQA

Notes

The normal is a dashed line drawn perpendicular to the surface at the point where the light ray meets the boundary.
The incident ray is the incoming light ray approaching the boundary.
The refracted ray is the ray that has changed direction inside the new medium.
The emergent ray is the ray that exits the second medium back into the original medium.
The angle of incidence and angle of refraction are both measured from the normal, not from the surface.

Drawing Ray Diagrams for Refraction

Draw the normal perpendicular to the glass surface at the point of incidence (where the ray hits the glass).
When entering a denser medium, draw the refracted ray bending towards the normal, roughly halfway between the original direction and the normal.
Continue the refracted ray across to the other side of the glass block.
At the second boundary, draw a new normal and bend the emergent ray away from the normal, as the ray is now entering a less dense medium.
Add the angle of incidence and angle of refraction to complete the diagram.

4. Drawing Ray Diagrams for RefractionAQA

Notes

Draw the normal perpendicular to the glass surface at the point of incidence (where the ray hits the glass).
When entering a denser medium, draw the refracted ray bending towards the normal, roughly halfway between the original direction and the normal.
Continue the refracted ray across to the other side of the glass block.
At the second boundary, draw a new normal and bend the emergent ray away from the normal, as the ray is now entering a less dense medium.
Add the angle of incidence and angle of refraction to complete the diagram.

Wavelength, Frequency, and Wave Speed

The wave speed equation states: v = f λ, where v is wave speed, f is frequency, and λ is wavelength.
When a wave refracts, its frequency always stays the same — only its wavelength changes.
If the wave speeds up (entering a less dense medium), its wavelength increases; if it slows down, its wavelength decreases.

5. Wavelength, Frequency, and Wave SpeedAQA

Notes

The wave speed equation states: v = f λ, where v is wave speed, f is frequency, and λ is wavelength.
When a wave refracts, its frequency always stays the same — only its wavelength changes.
If the wave speeds up (entering a less dense medium), its wavelength increases; if it slows down, its wavelength decreases.

Triangular Prisms and Dispersion

Different wavelengths of light are refracted by different amounts when passing through a medium.
White light contains all the wavelengths of visible light combined.
When white light passes through a triangular prism, each colour bends by a different degree, spreading out to form a spectrum similar to a rainbow — this is called dispersion.

6. Triangular Prisms and DispersionAQA

Notes

Different wavelengths of light are refracted by different amounts when passing through a medium.
White light contains all the wavelengths of visible light combined.
When white light passes through a triangular prism, each colour bends by a different degree, spreading out to form a spectrum similar to a rainbow — this is called dispersion.

Types of Waves: Longitudinal and Transverse

All waves transfer energy without transferring matter — oscillations are passed along rather than the particles themselves.
Longitudinal waves have oscillations parallel to the direction of energy transfer, with examples including sound waves and seismic P-waves (primary, faster waves).
In longitudinal waves, particles bunch together in regions called compressions and spread apart in regions called rarefactions.
Transverse waves have oscillations perpendicular to the direction of energy transfer, with examples including water surface waves, seismic S-waves (secondary, slower), and all electromagnetic waves.

7. Types of Waves: Longitudinal and TransverseAQA

Notes

All waves transfer energy without transferring matter — oscillations are passed along rather than the particles themselves.
Longitudinal waves have oscillations parallel to the direction of energy transfer, with examples including sound waves and seismic P-waves (primary, faster waves).
In longitudinal waves, particles bunch together in regions called compressions and spread apart in regions called rarefactions.
Transverse waves have oscillations perpendicular to the direction of energy transfer, with examples including water surface waves, seismic S-waves (secondary, slower), and all electromagnetic waves.

Wave Properties and Waveforms

A waveform is a graphical representation of a wave where the y-axis shows displacement (how far particles have moved from their equilibrium position).
The amplitude is the maximum displacement from the equilibrium (zero line), representing the peak of the wave.
When the x-axis shows distance, one complete wave gives the wavelength, symbol λ, measured in metres.
When the x-axis shows time, one complete wave gives the time period (T), which is the time taken for one complete wave to pass, measured in seconds.
Frequency is the number of complete waves passing a point per second, measured in hertz (Hz), and is the reciprocal of time period: f = 1 / T.

8. Wave Properties and WaveformsAQA

Notes

A waveform is a graphical representation of a wave where the y-axis shows displacement (how far particles have moved from their equilibrium position).
The amplitude is the maximum displacement from the equilibrium (zero line), representing the peak of the wave.
When the x-axis shows distance, one complete wave gives the wavelength, symbol λ, measured in metres.
When the x-axis shows time, one complete wave gives the time period (T), which is the time taken for one complete wave to pass, measured in seconds.
Frequency is the number of complete waves passing a point per second, measured in hertz (Hz), and is the reciprocal of time period: f = 1 / T.

The Wave Equation

The wave equation links wave speed, frequency, and wavelength: v = fλ, where v is in m/s, f in Hz, and λ in metres.
In a ripple tank, wavelength can be found by measuring the distance across 10 peaks and dividing by 10, then the wave equation gives the wave speed.
The speed of sound can be measured by timing how long a sound takes to echo off a wall a known distance away using a microphone and oscilloscope, then using speed = total distance / time.

9. The Wave EquationAQA

Notes

The wave equation links wave speed, frequency, and wavelength: v = fλ, where v is in m/s, f in Hz, and λ in metres.
In a ripple tank, wavelength can be found by measuring the distance across 10 peaks and dividing by 10, then the wave equation gives the wave speed.
The speed of sound can be measured by timing how long a sound takes to echo off a wall a known distance away using a microphone and oscilloscope, then using speed = total distance / time.

Sound Waves and Ultrasound

Sound waves cause the eardrum to vibrate, which is converted into a signal sent to the brain, and humans can hear frequencies between 20 Hz and 20,000 Hz (20 kHz).
Any frequency above 20 kHz is called ultrasound, which is beyond the range of human hearing.
Ultrasound is used in medical imaging by emitting pulses into the body and timing how long reflections take to return from different tissue boundaries, allowing a safe scan of unborn babies.
Sonar uses the same principle of timing reflected sound waves underwater to build up images of objects beneath a boat or around a submarine.

10. Sound Waves and UltrasoundAQA

Notes

Sound waves cause the eardrum to vibrate, which is converted into a signal sent to the brain, and humans can hear frequencies between 20 Hz and 20,000 Hz (20 kHz).
Any frequency above 20 kHz is called ultrasound, which is beyond the range of human hearing.
Ultrasound is used in medical imaging by emitting pulses into the body and timing how long reflections take to return from different tissue boundaries, allowing a safe scan of unborn babies.
Sonar uses the same principle of timing reflected sound waves underwater to build up images of objects beneath a boat or around a submarine.

Seismic Waves and the Earth's Structure

Seismic P-waves are longitudinal and can travel through both solid and liquid layers of the Earth.
Seismic S-waves are transverse and cannot travel through liquids, which is key evidence for the Earth having a molten outer core.
The absence of S-wave aftershocks on the opposite side of the Earth from an earthquake suggests there is a liquid region within the Earth's interior.

11. Seismic Waves and the Earth's StructureAQA

Notes

Seismic P-waves are longitudinal and can travel through both solid and liquid layers of the Earth.
Seismic S-waves are transverse and cannot travel through liquids, which is key evidence for the Earth having a molten outer core.
The absence of S-wave aftershocks on the opposite side of the Earth from an earthquake suggests there is a liquid region within the Earth's interior.

Reflection and Refraction of Light

Specular reflection occurs when light reflects off a smooth surface, such as a mirror, where the angle of incidence equals the angle of reflection, both measured from the normal.
Diffuse reflection occurs when light scatters off a rough surface, sending reflected rays in many different directions.
Refraction is the change in direction of a wave when it moves from one medium to another at an angle to the normal, caused by a change in wave speed.
When light moves from air into glass, it slows down and bends towards the normal, so the angle of refraction is smaller than the angle of incidence.

12. Reflection and Refraction of LightAQA

Notes

Specular reflection occurs when light reflects off a smooth surface, such as a mirror, where the angle of incidence equals the angle of reflection, both measured from the normal.
Diffuse reflection occurs when light scatters off a rough surface, sending reflected rays in many different directions.
Refraction is the change in direction of a wave when it moves from one medium to another at an angle to the normal, caused by a change in wave speed.
When light moves from air into glass, it slows down and bends towards the normal, so the angle of refraction is smaller than the angle of incidence.

Total Internal Reflection and Optical Fibres

The critical angle is the specific angle of incidence at which the angle of refraction is exactly 90°, and its value depends on the medium.
When the angle of incidence exceeds the critical angle, no light is refracted out and all light is reflected back inside the medium — this is total internal reflection.
Optical fibres (fibre optics) work by keeping light bouncing along a thin glass fibre through total internal reflection, allowing vast amounts of data to be transmitted very quickly.

13. Total Internal Reflection and Optical FibresAQA

Notes

The critical angle is the specific angle of incidence at which the angle of refraction is exactly 90°, and its value depends on the medium.
When the angle of incidence exceeds the critical angle, no light is refracted out and all light is reflected back inside the medium — this is total internal reflection.
Optical fibres (fibre optics) work by keeping light bouncing along a thin glass fibre through total internal reflection, allowing vast amounts of data to be transmitted very quickly.

Electromagnetic Waves and the EM Spectrum

Electromagnetic (EM) waves are transverse waves that do not require a medium and are the only waves that can travel through the vacuum of space.
The EM spectrum, in order of increasing frequency (and decreasing wavelength), is: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
EM waves are produced when electrons lose energy, except gamma rays which are emitted by atomic nuclei and therefore carry the most energy.
UV, X-rays, and gamma rays carry enough energy to ionise atoms by removing electrons, which can damage DNA and cause mutations leading to cancer.
All parts of the EM spectrum have practical uses including communications, cooking, heating, imaging, and medical treatments.

14. Electromagnetic Waves and the EM SpectrumAQA

Notes

Electromagnetic (EM) waves are transverse waves that do not require a medium and are the only waves that can travel through the vacuum of space.
The EM spectrum, in order of increasing frequency (and decreasing wavelength), is: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
EM waves are produced when electrons lose energy, except gamma rays which are emitted by atomic nuclei and therefore carry the most energy.
UV, X-rays, and gamma rays carry enough energy to ionise atoms by removing electrons, which can damage DNA and cause mutations leading to cancer.
All parts of the EM spectrum have practical uses including communications, cooking, heating, imaging, and medical treatments.

Lenses and Image Formation

A convex (converging) lens refracts parallel rays of light to meet at the principal focus, with the distance from the lens centre to this point called the focal length.
When an object is beyond the focal length of a convex lens, a real, inverted, and diminished image is formed where the two construction rays meet.
When an object is closer to a convex lens than the focal length, a virtual, upright, and magnified image is formed — this is how a magnifying glass works.
A concave (diverging) lens always causes rays to spread out, producing a virtual, upright, and diminished image.
Magnification is calculated as magnification = frac{image height}{text{object height}}, where a value greater than 1 means the image is larger than the object.

15. Lenses and Image FormationAQA

Notes

A convex (converging) lens refracts parallel rays of light to meet at the principal focus, with the distance from the lens centre to this point called the focal length.
When an object is beyond the focal length of a convex lens, a real, inverted, and diminished image is formed where the two construction rays meet.
When an object is closer to a convex lens than the focal length, a virtual, upright, and magnified image is formed — this is how a magnifying glass works.
A concave (diverging) lens always causes rays to spread out, producing a virtual, upright, and diminished image.
Magnification is calculated as magnification = frac{image height}{text{object height}}, where a value greater than 1 means the image is larger than the object.

Colour, Absorption, and Black Body Radiation

The colour of an object is determined by which wavelengths of light it reflects into our eyes, whilst absorbing all other wavelengths.
An object illuminated by a colour of light it absorbs will appear black, as no light is reflected — for example, a blue ball under red light appears black.
A black body is a theoretical object that perfectly absorbs and emits all wavelengths of radiation, a concept usefully applied to stars.
If an object absorbs radiation faster than it emits it, its temperature rises; as temperature increases, the rate of emission also increases until balance is reached.

16. Colour, Absorption, and Black Body RadiationAQA

Notes

The colour of an object is determined by which wavelengths of light it reflects into our eyes, whilst absorbing all other wavelengths.
An object illuminated by a colour of light it absorbs will appear black, as no light is reflected — for example, a blue ball under red light appears black.
A black body is a theoretical object that perfectly absorbs and emits all wavelengths of radiation, a concept usefully applied to stars.
If an object absorbs radiation faster than it emits it, its temperature rises; as temperature increases, the rate of emission also increases until balance is reached.