Dispersion is due to refraction.
In optics, dispersion is a phenomenon that causes the separation
of a wave into spectral components with different wavelengths, due
to a dependence of the wave's speed on its wavelength. It is most
often described in light waves, but it may happen to any kind of
wave that interacts with a medium or can be confined to a
waveguide, such as sound waves. Dispersion is sometimes called
chromatic dispersion to emphasize its wavelength-dependent
nature.
There are generally two sources of dispersion: material
dispersion, which comes from a frequency-dependent response of a
material to waves; and waveguide dispersion, which occurs when the
speed of a wave in a waveguide depends on its frequency. The
transverse modes for waves confined laterally within a finite
waveguide generally have different speeds (and field patterns)
depending upon the frequency (that is, on the relative size of the
wave, the wavelength, compared the size of the waveguide).
Dispersion in a waveguide used for telecommunication results in
signal degradation, because the varying delay in arrival time
between different components of a signal "smears out" the signal in
time. A similar phenomenon is modal dispersion, caused by a
waveguide having multiple modes at a given frequency, each with a
different speed. A special case of this is polarization mode
dispersion (PMD), which comes from a superposition of two modes
that travel at different speeds due to random imperfections that
break the symmetry of the waveguide.
