How is the refractive index of a medium related to the speed of light in it and in vacuum or air?

The refractive index of a medium is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.

$μ =\dfrac{\text{Speed of light in vacuum or air (c) }}{\text {Speed of light in that medium (V)}} \\[0.5em]$

Denser medium has a higher refractive index. Hence, the speed of light in such medium is lower in comparison to the speed of light in a medium which has a lower refractive index.