The refractive index of second medium with respect to the first medium $$n_{21}$$ is given by which formula?

A

The sum of the speed of light in the first medium and the speed of light in the second medium $$v_1 + v_2$$

B

The product of the speed of light in first medium and the speed of light in the second medium. $$v_1 \times v_2$$

C

Ratio of speed of light in first medium to speed of light in second medium. $$v_1 / v_2$$

D

Ratio of speed of light in second medium to speed of light in first medium. $$v_2 / v_1$$