PLZ ANSWER

we would like to understand where you are getting stuck on this question, once we are clear, we would be able to explain things better.

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