Here's why:

* Refractive index is a measure of how much a material slows down light compared to its speed in a vacuum.

* When light enters a material, it interacts with the atoms in the material. This interaction causes the light to slow down.

* The more dense the material, the more atoms it has packed together, leading to more interactions and a greater slowing of light. This is why light travels slower in water (higher refractive index) than in air (lower refractive index).