The light rays travel from a glass slab with refractive index $μ_1$, to the second slab with refractive index $μ_2$. The refractive index of the second slab with respect to the first one is:

(Assume that the two slabs have the same thickness)

$1μ_2 = μ_2/μ_1$

The correct answer is Option (2) → $1μ_2 = μ_2/μ_1$

$\text{Given: Refractive indices } \mu_1 \text{ (first medium)},~ \mu_2 \text{ (second medium)}$

$\text{Refractive index of second medium with respect to first: } \mu_{21} = \frac{\text{speed of light in first medium}}{\text{speed of light in second medium}}$

$\text{But } \mu = \frac{c}{v} \Rightarrow v_1 = \frac{c}{\mu_1},~ v_2 = \frac{c}{\mu_2}$

$\mu_{21} = \frac{v_1}{v_2} = \frac{c/\mu_1}{c/\mu_2} = \frac{\mu_2}{\mu_1}$

$\text{Answer: } \mu_{21} = \frac{\mu_2}{\mu_1}$