The frequency of a wave is the number of waves passing a point each second.

The idea

Frequency counts complete waves rather than measuring a distance: it is the number of waves passing a fixed point every second, and its unit is the hertz, where 1 Hz is one wave per second. In a ripple tank you count the waves crossing a mark over a measured time and divide, so 40 waves in 10 s gives 4 Hz. Frequency is set by the source, not by the medium, so it is unchanged when a wave crosses a boundary. It is the reciprocal of the period, f = 1 ÷ T, and it appears in wave speed = frequency × wavelength. A higher frequency sound is heard as a higher pitch. Frequencies cover an enormous range: a bass note is around 50 Hz, the upper limit of human hearing about 20 000 Hz, and visible light of the order of 10¹⁴ Hz.

What earns the marks

• Frequency as the number of complete waves passing a point each second.
• The unit hertz, Hz.
• Counting waves over a measured time and dividing by that time.
• Using f = 1 ÷ T to convert a period into a frequency.

Common mistakes and how to put them right

• Counting the waves in 10 s and quoting the count itself as the frequency.

• Confusing frequency with wave speed, so a higher frequency is taken to mean a faster wave.

• Giving frequency the unit seconds because it involves time.

• Assuming frequency changes when a wave moves into a new medium.

Exam tips

• Count waves over ten seconds rather than one, then divide; it reduces the timing uncertainty.
• Write 1 ÷ T rather than T when converting; the reciprocal step is where marks are lost.
• Give the unit Hz with every frequency answer, including ones read from a graph.