Describing Wave Motion (Edexcel GCSE Combined Science): Revision Note

Describing Wave Motion

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

  • amplitude

  • wavelength

  • frequency

  • time period

  • wave velocity

  • wavefront

Amplitude

• Amplitude is defined as:

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

• It is given the symbol 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:

  • 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) 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

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 and is measured in hertz (Hz)

Time period

• The time period of a wave is defined as:

The time taken for a single wave to pass a point

• Time period is given the symbol and is measured in seconds (s)

• The frequency and period of a wave are related by the equation:

• Where:

  • = frequency of the wave, measured in hertz (Hz)

  • = period of the wave, measured in seconds (s)

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

Using a graph to determine time period

Diagram showing the time period of a wave

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

Wavefront

• Wavefronts are a useful way of picturing waves from above

• Each wavefront, drawn as a single line, 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 as viewed from above

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