Here's why:

* Refractive index is a measure of how much light bends when it passes from one medium to another.

* A higher refractive index means the light bends more, and therefore travels slower in that material.

* This is because the light interacts more strongly with the atoms of the material, causing it to slow down.

Example:

* Light travels faster in air than in water because water has a higher refractive index than air.

* Light travels even slower in glass than in water because glass has a higher refractive index than water.