Question

what is meant by refractive index?

Answer 1

it is the ratio of the sin of angle of incidence and the sin of angle of refraction ............
    given by      μ=sin i/sin r

Answer 2

There are two laws of refraction. Since Refractive index is associated with the second law I am mentioning that only. The second law of of refraction states that :
2. For any given pair of media the ratio of sine of angle of incidence and angle of refraction is a constant μ .
        $\frac{sin i}{sin r}$ = μ
The second law is also known as Snell's law. Where μ is the refractive index of the first medium with respect to the second medium. μ is spelled as Mew.