CIE IGCSE Physics

Revision Notes

3.1.1 Features of Waves

Test Yourself

Waves - Basic

  • Waves transfer energy and information
  • 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

  • In all cases, waves transfer energy without transferring matter
    • For water waves, this means it is the wave and not the water (the matter) itself that travels
    • 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

Worked example

The diagram below shows a toy duck bobbing up and down on top of the surface of some water, as waves pass it underneath.

Use this image

Explain how the toy duck demonstrates that waves do not transfer matter.

Step 1: Identify the type of wave

    • The type of wave on the surface of a body of water is a transverse wave
    • This is because the duck is moving perpendicular to the direction of the wave

Step 2: Describe the motion of the toy duck

    • The plastic duck moves up and down but does not travel with the wave

Step 3: Explain how this motion demonstrates that waves do not transfer matter

    • Both transverse and longitudinal waves transfer energy, but not the particles of the medium
    • This means when a wave travels between two points, no matter actually travels with it, the points on the wave just vibrate back and forth about fixed positions
    • Objects floating on the water simply bob up and down when waves pass under them, demonstrating that there is no movement of matter in the direction of the wave, only energy

Exam Tip

There is a key distinction between the particles (or oscillations) of a wave, and the wave itself.

The motion of the wave causes the particles to move. The particles themselves are not the wave. 

Wave Motion

  • Wave vibrations can be shown on ropes (transverse) and springs (longitudinal)

Wave on rope and spring, downloadable AS & A Level Physics revision notes

Use this image

Waves can be shown through vibrations in ropes or springs

Demonstrating Wave Motion

  • Properties of waves, such as frequency, wavelength and wave speed, can be observed using water waves in a ripple tank

Ripple tank, IGCSE & GCSE Physics revision notes

Use this image

Wave motion of water waves may be demonstrated using a ripple tank

  • The wavelength of the waves can be determined by:
    • Using a ruler to measure the length of the screen
    • Dividing this distance by the number of wavefronts
  • The frequency can be determined by:
    • Timing how long it takes for a given number of waves to pass a particular point
    • Dividing the number of wavefronts by the time taken
  • The wave speed can then be determined by:
    • Using the equation wave speed = frequency × wavelength

Features of a Wave

  • When describing wave motion, there are several terms which are important to know, including:
    • Crest (Peak)
    • Trough
    • Amplitude
    • Wavelength
    • Frequency
    • Wave speed
    • Wavefront

Crests & Troughs

  • A crest, or a peak, is defined as:

The highest point on a wave above the equilibrium, or rest, position

  • A trough is defined as

The lowest point on a wave below the equilibrium, or rest, position

Transverse wave diagram, downloadable AS & A Level Physics revision notes

Use this image

Diagram showing a crest and a trough on a transverse wave

Amplitude

  • Amplitude is defined as:

The distance from the undisturbed position to the peak or trough of a wave

  • It is given the symbol A and is measured in metres (m)
  • Amplitude is the maximum or minimum displacement from the undisturbed position

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:
    • 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

  • The wavelength is given the symbol λ (lambda) and is measured in metres (m)
  • The distance along a wave is typically put on the x-axis of a wave diagram

Amplitude and wavelength, downloadable AS & A Level Physics revision notes

Use this image

Diagram showing the amplitude and wavelength of a wave

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)

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

  • Wave speed is given the symbol, ν, and is measured in metres per second (m/s), it can be calculated using:

wave speed = frequency × wavelength

Wavefront

  • Wavefronts are a useful way of picturing waves from above: each wavefront is used to represent a single wave
  • 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

series-of-wavefronts, IGCSE & GCSE Physics revision notes

Use this image

Diagram showing a wave moving to the right, drawn as a series of wavefronts

Worked example

Small water waves are created in a ripple tank by a wooden bar. The wooden bar vibrates up and down hitting the surface of the water. The diagram below shows a cross-section of the ripple tank and water.

Use this image

Identify the letter which shows:

a)
The amplitude of a water wave.
b)
The wavelength of the water wave.

Part (a)

Step 1: Recall the definition of amplitude

    • Amplitude = The distance from the undisturbed position to the peak or trough of a wave

Step 2: Mark the undisturbed position on the wave

    • This is the centre of the wave

WE Ripple Tank Ans 1, downloadable IGCSE & GCSE Physics revision notes

Use this image

Step 3: Identify the arrow between the undisturbed position and a peak

    • The amplitude is shown by arrow D

Part (b)

Step 1: Recall the definition of wavelength

    • Wavelength = The distance from one point on the wave to the same point on the next wave

Step 2: Draw lines on each horizontal arrow

    • This helps to identify the points on the wave the arrows are referring to

WE Ripple Tank Ans 2, downloadable IGCSE & GCSE Physics revision notes

Use this image

Step 3: Identify the arrow between two of the same points on the wave

    • The wavelength is shown by arrow C

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