Different materials possess different values of refractive indices. The materials which are optically transparent - which let light through them, have the refractive indices.
The index depends on the material properties, its density, chemical properties , the resistance it offers to light to pass through etc.
Also, crown glass has 1.5+ and flint glass : 1.6+ and water has 1.33.
The refractive index affects the speed of light and the wavelength of light when light passes thru that medium. Speed of light reduces by the factor of refractive index and wave length increases by the same factor.
For different colors of light the refractive index varies by a little amount too.
