Properties of waves: identify amplitude and wavelength from given diagrams

The idea

Amplitude is the maximum displacement of a point on the wave from its rest position, measured in metres; on a displacement–distance graph it is the vertical distance from the centre line to a crest or trough. Wavelength is the distance between two successive crests, two successive troughs, or any two equivalent points on consecutive waves, also measured in metres.

For example: If a diagram shows a crest 0.20 m above the centre line and crests 1.50 m apart, the amplitude is 0.20 m and the wavelength is 1.50 m.

What earns the marks

• Read amplitude as the vertical distance from the rest position to a crest or to a trough.
• Read wavelength as the horizontal distance between two successive crests or two successive troughs.
• Use equivalent points on consecutive waves, such as trough to trough, when crests are not convenient.
• State both quantities with the correct unit, the metre, and an appropriate number of significant figures.
• Recognise that the peak-to-trough height is twice the amplitude, not the amplitude itself.

Common mistakes and how to put them right

• Measuring amplitude from trough to crest.

  Put it right: Measure from the rest position to one crest or one trough.

• Measuring wavelength from a crest to the next trough.

  Put it right: Measure between equivalent points on successive waves.

• Omitting units or writing them incorrectly.

  Put it right: Give amplitude and wavelength in metres, for example 0.20 m and 1.50 m.

Exam tips

• Mark the rest position with a dashed horizontal line before measuring amplitude.
• Count the number of complete waves shown and divide the total distance by that number to find wavelength if only one value is labelled.
• Check whether the diagram shows displacement against distance or against time, because wavelength is read from a distance axis.