travelling through a medium with a frequency-dependent refractive index 
by 
where 
is the speed of light in a vacuum.
of: a given type of wave in a specified medium
is: an expression which describes the variation of the wave's wavelength (or some related quantity such as wavenumber) with the frequency of the wave. [P10.3]
is exemplified: for an electromagnetic wave of wavelength travelling through a medium with a frequency-dependent refractive index by where is the speed of light in a vacuum.
is also exemplified: by the dispersion relation for the de Broglie wave of a free particle 
where 
is the angular frequency and 
is the corresponding angular wavenumber.
Copyright 1997, The Open University