Solution : Here, v

_{1}= 1.5 × 10^{8}m/s and v_{2}= 3 x 10^{8}m/s.
We know that, the refractive index with respect to air is

_{2}η

_{1}= v

_{2}÷ v

_{1}= (3 x 10

^{8}) ÷ (1.5 × 10

^{8 }) = 2

Thus, the refractive index of the medium with respect to air is 2.