Features of Waves (Cambridge O Level Physics)

• 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 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

• 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