Question

Define refractive index in terms of a speed of light in two media

Answer 1

refractive index is the change in speed of light on going from one medium to another.

Answer 2

well, if I say, refractive index is simply the ratio between the speed of light in vacuum(or air) to the speed of light in another medium(generally denser). Since, the speed of light is maximum in vacuum(or air), whenever light passes through any other medium which is denser, then at that time light experiences resistance due to presence of matter. So it's speed decreases. Refractive index, in simple words, means the no. of times light will travel faster in vacuum(or air) than that in a certain denser medium. No medium can have refractive index less than 1, since no medium has the capability to pass light at a greater speed than that in vacuum(or air).

Hope it clears your confusion.