what is refractive index??

The ratio of the sine of the angle of incidence to the sine of anlgle of refraction is a constant and called refractive index of that material wrt to other material

                                              sin i/sin r = constant. = ₁n₂

This is called refractive index of medium of 2 with respect of medium 1

This can be explaned on the basis of speed fo light which is different in different media 

sin i/sin r = v1/v2= speed in first medium/ speed in second medium