Editing Dispersion relation (section)

===Waves and optics===
{{Further|Dispersion (optics)}}
The name "dispersion relation" originally comes from [[optics]]. It is possible to make the effective speed of light dependent on wavelength by making light pass through a material which has a non-constant [[index of refraction]], or by using light in a non-uniform medium such as a [[waveguide]]. In this case, the waveform will spread over time, such that a narrow pulse will become an extended pulse, i.e., be dispersed. In these materials, <math>\frac{\partial \omega}{\partial k}</math> is known as the [[group velocity]]<ref>{{cite book| author=F. A. Jenkins and H. E. White |date=1957|title=Fundamentals of optics| url=https://archive.org/details/fundamentalsofop00jenk | url-access=registration |publisher=McGraw-Hill|location=New York| page=[https://archive.org/details/fundamentalsofop00jenk/page/223 223]| isbn=0-07-032330-5}}</ref> and corresponds to the speed at which the peak of the pulse  propagates, a value different from the [[phase velocity]].<ref>{{cite book|author= R. A. Serway, C. J. Moses and C. A. Moyer |year=1989| title=Modern Physics| publisher=Saunders|location=Philadelphia|page= 118|isbn= 0-534-49340-8}}</ref>