Question

what is meant by the refractive index of a medium with respect to air

Answer 1

Refractive index of a medium with respect to air is the ratio of the velocity of light in air to the velocity of the light in the medium.

It is also the ratio of the refractive index of the medium with respect to vacuum to the refractive index of air with respect to vacuum.

Answer 2

it means that the ratio
refractive index of air is to the refractive index of vaccum