Students should be able to describe wave motion in terms of their…

The idea

Wave motion is described by four linked quantities. Amplitude is the maximum displacement of a point from its undisturbed position, measured in metres. Wavelength is the distance between two identical points on adjacent waves, such as crest to crest, also in metres. Frequency is the number of complete waves passing a point each second, measured in hertz (Hz). Period is the time for one complete wave to pass a point, measured in seconds, and is the reciprocal of frequency: T = 1 ÷ f. For example, a wave of frequency 5 Hz has period 0.2 s. On a displacement–distance graph, wavelength is read along the x-axis; on a displacement–time graph, period is read along the x-axis. Amplitude is read from the vertical axis on either graph.

What earns the marks

• Amplitude is the maximum displacement of a point on a wave from its undisturbed position, measured in metres.
• Wavelength is the distance between two identical points on adjacent waves, for example crest to crest or trough to trough, measured in metres.
• Frequency is the number of complete waves passing a fixed point each second, measured in hertz (Hz).
• Period is the time taken for one complete wave to pass a fixed point, measured in seconds.
• Frequency and period are reciprocals: f = 1 ÷ T and T = 1 ÷ f, so a 4 Hz wave has period 0.25 s.
• On a displacement–distance graph, wavelength is read along the horizontal axis; on a displacement–time graph, period is read along the horizontal axis.

Common mistakes and how to put them right

• Confusing wavelength with amplitude.

  Put it right: Wavelength is a horizontal distance between identical points on adjacent waves, while amplitude is a vertical maximum displacement from the undisturbed position.

• Treating frequency and period as the same quantity.

  Put it right: Frequency is waves per second in Hz, whereas period is seconds per wave; they are reciprocals, not equal.

• Reading period from a displacement–distance graph.

  Put it right: Period must be read from a displacement–time graph, while wavelength is read from a displacement–distance graph.

Exam tips

• State the quantity, its meaning and its unit in one sentence, for example 'frequency is the number of waves per second, measured in hertz'.
• When using T = 1 ÷ f, write the equation, substitute the value with its unit, then give the answer with the correct unit, such as s or Hz.
• Label axes carefully before reading values: check whether the horizontal axis is distance (read wavelength) or time (read period).