How is the refractive index of a material related to the speed of light in air?

A

Refractive index $$=$$ Speed of light in air $$+$$ Speed of light in material

B

Refractive index $$=$$ Speed of light in air $$/$$ Speed of light in material

C

Refractive index $$=$$ Speed of light in air $$\times$$ Speed of light in material

D

Refractive index $$=$$ Speed of light in material $$/$$ Speed of light in air