UT Testing-Section 2 Physics of Ultrasound

Dispersion refers to the fact that in a real medium such as water, air, or glass,
a wave traveling through that medium will have a velocity that depends upon
its frequency. Dispersion occurs for any form of wave, acoustic,
electromagnetic, electronic, even quantum mechanical. Dispersion is
responsible for a prism being able to resolve light into colors and defines the
maximum frequency of broadband pulses one can send down an optical fiber
or through a copper wire. Dispersion affects wave and swell forecasts at
sea and influences the design of sound equipment. Dispersion is a physical
property of the medium and can combine with other properties to yield very
strange results. For example, in the propagation of light in an optical fiber, the
glass introduces dispersion and separates the wavelengths of light according
to frequency, however if the light is intense enough, it can interact with the
electrons in the material changing its refractive index. The combination of
dispersion and index change can cancel each other leading to a wave that
can propagate indefinitely maintaining a constant shape. Such a wave has
been termed a soliton.
http://www.rpi.edu/dept/chem-eng/WWW/faculty/plawsky/Comsol%20Modules/DispersiveWave/DispersiveWave.html