Wave behaviour: define wavelength and frequency

The idea

Wavelength and frequency describe two different features of a wave. Wavelength is the distance between two matching points on consecutive waves, such as from one crest to the next crest, or one compression to the next compression in a sound wave; it is measured in metres (m). Frequency is the number of complete waves passing a fixed point each second; it is measured in hertz (Hz), where 1 Hz means one wave per second. A high-frequency wave repeats often and has many cycles per second; a long-wavelength wave has crests far apart. The two are linked through wave speed by v = f × λ, so for a fixed speed a higher frequency means a shorter wavelength.

What earns the marks

• Defines wavelength as the distance between corresponding points on successive waves, for example crest to crest, measured in metres.
• Defines frequency as the number of complete waves passing a point per second, measured in hertz.
• Distinguishes the two quantities, recognising that wavelength is a distance and frequency is a rate.
• Links the definitions to the correct units, metres for wavelength and hertz for frequency.

Common mistakes and how to put them right

• Describes wavelength as the height of the wave.

  Put it right: State that it is the distance between matching points on successive waves.

• Defines frequency as the time for one wave.

  Put it right: Describe it as the number of waves per second instead.

• Gives frequency in metres or wavelength in hertz.

  Put it right: Match each quantity to its own unit.

• Confuses amplitude with wavelength.

  Put it right: Keep amplitude as the maximum displacement from the rest position.

Exam tips

• Use the phrase 'per second' when defining frequency to make the rate clear.
• Name the matching points you measure between, such as crest to crest, when defining wavelength.
• Check the unit attached to each definition before selecting an answer.