Features of Waves (Cambridge (CIE) O Level Physics): Revision Note

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

Wave Motion

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

Motion of Transverse and Longitudinal Waves

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

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

Crest and Trough of a Wave

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

Wavelength and Amplitude

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

Wavefronts

• 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

Wavefronts

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