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# Long description

This figure shows a graph. The x-axis is labelled distance, but no tick marks or units are shown. The y-axis is unlabelled apart from an arrow at the top.

A red sine curve – that is, a smoothly curved line rising and falling regularly – starts at the origin, and rises first to a high point labelled ‘peak’. It then falls to an equal distance below the x-axis, labelled trough, and rises again to another peak, and falls to another trough and so on. There are three peaks and two troughs.

Amplitude is shown in three places on the graph. First, a dashed line has been drawn from the first peak back to the y-axis, horizontal to the x-axis, and the distance from the origin to the point where the dashed line crosses the y-axis is labelled ‘amplitude’. Second, the distance from the x-axis to the low point that represents the second trough is also labelled ‘amplitude’. Third, the distance from the x-axis to the top of the third peak is labelled ‘amplitude’. These measurements are all identical in length.

Wavelength is also illustrated on the graph in several places. The distance from the first peak to the second peak is labelled ‘wavelength’. The distance from the first trough to the second trough is labelled ‘wavelength’. Also, a number of dashed lines run horizontally, each joining a point the curve falling from the first peak to the first trough with an equivalent point on the curve joining the second peak to the second trough. These dashed lines are labelled ‘alternative measurements of wavelength’. These measurements are all identical in length.