What is dispersion?
Dispersion within optics consider as the change in phase velocity of an electromagnetic wave along with wavelength. Conversely, when there is a variation of refractive index of the material along with wavelength this leads to dispersion and is termed as material dispersion. In many materials the refractive index reduces with increase into wavelength, λ, and therefore (dn/dλ)<0.
Generally, dispersion can occur because of material properties or because of the geometry of the structure through that the electromagnetic wave is propagating that is the waveguide. Hence, based onto the source of dispersion, this is classified in material dispersion and waveguide dispersion.