Features of Waves (Cambridge (CIE) IGCSE Physics): Revision Note

Waves & energy transfer

• Waves transfer energy without transferring matter

  • For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels

• Objects floating on water provide evidence that waves only transfer energy and not matter 

  • It is possible to see objects on the surface of the water bob up and down but not change their position

  • This is because it is the wave, and not the water (the matter), that travels

• Waves are described as oscillations or vibrations about a fixed point

  • For example, ripples cause particles of water to oscillate up and down

  • Sound waves cause particles of air to vibrate back and forth

Wave motion

• Wave motion can be illustrated by:

  • vibrations in ropes and springs

  • experiments using water waves

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• Properties of waves can be observed using water waves in a ripple tank (opens in a new tab)

  • These properties include frequency, wavelength, amplitude and wave speed

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Features of a wave

• When describing wave motion, there are several terms which are important to know, including:

  • wavefront

  • amplitude

  • wavelength

  • frequency

  • crest (peak)

  • trough

  • wave speed

Wavefront

• Wavefronts are a useful way of picturing waves from above

• Each wavefront, drawn as a single line, joins points on the wave that are in step with each other, which in practice are usually the crests

• The image below illustrates how wavefronts are visualised:

  • The arrow shows the direction the wave is moving and is sometimes called a ray

  • The space between each wavefront represents the wavelength

  • When the wavefronts are close together, this represents a wave with a short wavelength

  • When the wavefronts are far apart, this represents a wave with a long wavelength

Wavefronts as viewed from above

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Amplitude

• Amplitude is defined as:

The maximum displacement of a wave from its undisturbed position

• It is given the symbol $A$ and is measured in metres (m)

• On a graph where the vertical axis is displacement, amplitude is measured from the undisturbed position to either the highest point of the wave (peak) or the lowest point (trough)

Wavelength

• Wavelength is defined as:

The distance from one point on the wave to the same point on the next wave

• In a transverse wave (opens in a new tab):

  • The wavelength can be measured from one peak to the next peak

• In a longitudinal wave:

  • The wavelength can be measured from the centre of one compression to the centre of the next

• Wavelength is given the symbol $\lambda$ (lambda) and is measured in metres (m)

• On a graph where the horizontal axis is distance, the wavelength can be determined by measuring the distance from one point on the wave to the same point on the next wave

Using a graph to determine wavelength and amplitude

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Frequency

• Frequency is defined as:

The number of waves passing a point in a second

• Frequency is given the symbol $f$ and is measured in hertz (Hz)

Crests and troughs

• A crest, or a peak, is defined as:

The highest point on a wave above its undisturbed position

• A trough is defined as:

The lowest point on a wave below its undisturbed position

Wave crests and troughs

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Wave speed

• Wave speed is the speed at which energy is transferred through a medium

• Wave speed is defined as:

The distance travelled by a wave each second

• The equation used to calculate wave speed is explained in The wave equation