Index of refraction are given relative to some medium, usually vacuum if the other medium is not stated. According to the definition of index of refraction we have that
where [itex]c[/itex] is the velocity of light in vacuum and [itex]v[/itex] is the velocity of light in the medium of which the refractive index is given. Refraction is therefore a result of the velocity of light changing as it passes from one medium (vacuum) to the other (eg. glass). If the refraction index of some medium is therefore [itex]1.0[/itex] no refraction will take place. When light enters mediums other than vacuum it is slowed down, therefore the above ratio gives values larger than one. Mediums with larger indices of refraction therefore slows light down more than ones with smaller indices of refraction resulting in light being refracted more in mediums with larger refractive indices. Note that this slowing down of light in medium is also color dependent - higher frequencies (shorter wavelenghts) is slowed down more than lower frequencies (longer wavelengths) of light. The refractive index is therfore also dependent on the wavelength of the light entering the medium.