The refractive index of the second medium with respect to the first medium, 2n1 = \(\frac{sin\,i}{sin\,r}=\frac{v_1}{v_2}\)
where i is the angle of incidence, r is the angle of refraction (as the ray of light passes obliquely from the first medium to the second medium), v1 is the magnitude of the velocity (speed) of light in the first medium and v2 is the magnitude of the velocity of light in the second medium.
