<P> In the special case of dispersion - free and uniform media, waves other than sinusoids propagate with unchanging shape and constant velocity . In certain circumstances, waves of unchanging shape also can occur in nonlinear media; for example, the figure shows ocean waves in shallow water that have sharper crests and flatter troughs than those of a sinusoid, typical of a cnoidal wave, a traveling wave so named because it is described by the Jacobi elliptic function of m - th order, usually denoted as cn (x; m). Large - amplitude ocean waves with certain shapes can propagate unchanged, because of properties of the nonlinear surface - wave medium . </P> <P> If a traveling wave has a fixed shape that repeats in space or in time, it is a periodic wave . Such waves are sometimes regarded as having a wavelength even though they are not sinusoidal . As shown in the figure, wavelength is measured between consecutive corresponding points on the waveform . </P> <P> Localized wave packets, "bursts" of wave action where each wave packet travels as a unit, find application in many fields of physics . A wave packet has an envelope that describes the overall amplitude of the wave; within the envelope, the distance between adjacent peaks or troughs is sometimes called a local wavelength . An example is shown in the figure . In general, the envelope of the wave packet moves at a different speed than the constituent waves . </P> <P> Using Fourier analysis, wave packets can be analyzed into infinite sums (or integrals) of sinusoidal waves of different wavenumbers or wavelengths . </P>

What is the unit of wavelength of light