Define and use the terms frequency and wavelength as applied to waves

The idea

Frequency is the number of complete waves passing a fixed point each second, measured in hertz (Hz); one hertz means one wave per second. Wavelength is the distance between two corresponding points on consecutive waves, such as from one crest to the next crest, measured in metres. On a displacement–distance graph the horizontal axis shows position, so the distance for one complete cycle gives the wavelength directly. On a displacement–time graph the horizontal axis shows time, so one complete cycle gives the period, and frequency is the reciprocal of the period (f = 1/T). Use these definitions to read values from graphs and to relate frequency, wavelength and wave speed.

What earns the marks

• Defines frequency as the number of complete waves passing a point each second, measured in hertz.
• Defines wavelength as the distance between corresponding points on consecutive waves, measured in metres.
• Reads wavelength from a displacement–distance graph as the length of one complete cycle.
• Reads period from a displacement–time graph and calculates frequency as the reciprocal of the period.
• Applies the terms correctly when comparing waves of different frequency or wavelength.

Common mistakes and how to put them right

• Reads wavelength from a displacement–time graph.

  Put it right: Recognise that a displacement–time graph gives the period, from which frequency is found.

• Reads frequency directly from a displacement–distance graph.

  Put it right: Recognise that this graph gives wavelength, not frequency.

• Measures wavelength between a crest and the next trough.

  Put it right: Measure between corresponding points such as crest to crest.

• Gives frequency in metres or wavelength in hertz.

  Put it right: Keep frequency in hertz and wavelength in metres.

Exam tips

• Check which quantity is on the horizontal axis before reading a value from a wave graph.
• Remember that frequency equals one divided by the period when working from a displacement–time graph.
• Use corresponding points, such as crest to crest, when identifying one complete wavelength.