Here's why:

* Index of refraction (n) is a measure of how much light slows down when it enters a material compared to its speed in a vacuum.

* Vacuum: Light travels at its fastest speed in a vacuum, approximately 299,792,458 meters per second (often rounded to 3 x 10^8 m/s).

* Materials: When light enters a material, it interacts with the atoms of that material, which slows it down. The more dense the material, the more interactions, and the slower the light travels.

Therefore:

* A material with a higher index of refraction means light slows down more significantly.

* A material with a lower index of refraction means light slows down less significantly.

Example:

* Air: Index of refraction ~ 1.0003 (almost the same as a vacuum)

* Water: Index of refraction ~ 1.33

* Diamond: Index of refraction ~ 2.42

Light travels slower in water than in air, and even slower in diamond than in water. This difference in speed is responsible for phenomena like refraction (bending of light) and the sparkle of diamonds.