Transverse and longitudinal waves

Waves may be either transverse or longitudinal.

The ripples on a water surface are an example of a transverse wave.

Longitudinal waves show areas of compression and rarefaction. Sound waves travelling through air are longitudinal.

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Students should be able to describe the difference between longitudinal and transverse waves.

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Students should be able to describe evidence that, for both ripples on a water surface and sound waves in air, it is the wave and not the water or air itself that travels.

Properties of waves

Students should be able to describe wave motion in terms of their amplitude, wavelength, frequency and period.

The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.

The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave.

The frequency of a wave is the number of waves passing a point each second.

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\(\text{period} = \dfrac{1}{\text{frequency}}\)

\(T = \dfrac{1}{f}\)

period, T, in seconds, s
frequency, f, in hertz, Hz

The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.

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All waves obey the wave equation:

\(\text{wave speed}  =  \text{frequency}  ×  \text{wavelength} \)

\(v = f \lambda \)

wave speed, v, in metres per second, m/s
frequency, f, in hertz, Hz
wavelength, λ, in metres, m

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Students should be able to:
- identify amplitude and wavelength from given diagrams
- describe a method to measure the speed of sound waves in air
- describe a method to measure the speed of ripples on a water surface.

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Students should be able to show how changes in velocity, frequency and wavelength, in transmission of sound waves from one medium to another, are inter-related.

Reflection of waves

Waves can be reflected at the boundary between two different materials.

Waves can be absorbed or transmitted at the boundary between two different materials.

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Students should be able to construct ray diagrams to illustrate the reflection of a wave at a surface.

Students should be able to describe the effects of reflection, transmission and absorption of waves at material interfaces.

Sound waves

Sound waves can travel through solids causing vibrations in the solid.

Within the ear, sound waves cause the ear drum and other parts to vibrate which causes the sensation of sound. The conversion of sound waves to vibrations of solids works over a limited frequency range. This restricts the limits of human hearing.

Students should be able to:
- describe, with examples, processes which convert wave disturbances between sound waves and vibrations in solids. Examples may include the effect of sound waves on the ear drum
- explain why such processes only work over a limited frequency range and the relevance of this to human hearing.

Students should know that the range of normal human hearing is from 20 Hz to 20 kHz.

Waves for detection and exploration

Students should be able to explain in qualitative terms, how the differences in velocity, absorption and reflection between different types of wave in solids and liquids can be used both for detection and exploration of structures which are hidden from direct observation.

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Ultrasound waves have a frequency higher than the upper limit of hearing for humans. Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. This allows ultrasound waves to be used for both medical and industrial imaging.

Seismic waves are produced by earthquakes. P-waves are longitudinal, seismic waves. P-waves travel at different speeds through solids and liquids. S-waves are transverse, seismic waves. S-waves cannot travel through a liquid. P-waves and S-waves provide evidence for the structure and size of the Earth’s core.

Echo sounding, using high frequency sound waves is used to detect objects in deep water and measure water depth.

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Students should be aware that the study of seismic waves provided new evidence that led to discoveries about parts of the Earth which are not directly observable.