The refractive index of any medium gives the ratio of the speed of light in vacuum to the speed of light in that medium.
For example, the refractive index of water, nw = 1.33.
This means that the ratio of the speed of light in vacuum or air to the speed of light in water is 1.33.
The refractive index of a medium is dependent on the following factors:
(i) Nature of the medium,
(ii) Nature of the surrounding medium,
(iii) Wavelength of light used, and
(iv) Temperature.