A single full wavelength (\(\lambda\)) is measured between two points on the wave that are in the same phase (for example, from crest to the next consecutive crest, or from zero-crossing with the same slope to the next such zero-crossing). From the diagram:

• \(A\) and \(E\) are successive zero-crossings with the same slope, so \(AE = \lambda\).
• \(B\) and \(D\) appear to be a crest and the next trough (or vice versa), which are separated by half a wavelength \(= \frac{\lambda}{2}\).
• The segments \(AB\) or \(DE\) each represent a quarter wavelength \(\frac{\lambda}{4}\), because they go from a zero-crossing to an extreme (crest or trough) or from an extreme back to a zero-crossing.

Hence, the distance that corresponds to half the wavelength is BD.