Question

What is the meaning of refractive index of medium 2 with respect to medium 1

Answer 1

The refractive index of a medium with respect to another medium is defined as the ratio of speed of light in the first medium to the ratio of speed of light in second medium.
Let v1 and v2 be the speeds of light in first and second media respectively.
THEN
refractive index of second medium with respect to  first is given by
n₂₁=speed of light in medium1/speed of light in medium 2
n₂₁=v1/v2
dividing numerator and denominator by c, we get
n₂₁=n2/n1
This is called as relative refractive index.
Relative refractive index= n₂₁=Refractive index of second medium/                                                                refractive index of first medium