The refractive index or phase index is how much slower than the vacuum speed of light, c, the light will travel in a material. This speed is the phase velocity. It depends on both the type of material and the color of the light.
The refractive, (phase), index, n, of a material relates the vacuum wavelength l0 to the wavelength in the medium, l.
This means that the phase velocity, vp, of light in a medium is lower than the speed of light, c. For monochromatic sources where dispersion is not significant the velocity of light in the fiber can be considered to be related directly to the refractive index.