Light travels slower in high density materials due to the increased interaction of light waves with the atoms and molecules present in the medium. As light passes through a material, it interacts with the electrons surrounding the atoms and molecules, causing a phenomenon called refraction. This interaction results in a decrease in the speed of light compared to its speed in a vacuum.

The refractive index of a material, which is a measure of how much light slows down when passing through that material, is directly related to the material's density. Generally, denser materials have a higher refractive index, meaning that light travels slower through them.

For instance, the refractive index of water is approximately 1.33, while the refractive index of glass is around 1.5. This indicates that light travels slower in glass than in water due to the higher density of glass. Similarly, the refractive index of diamond, which is one of the densest naturally occurring materials, is about 2.42, resulting in a significant slowdown of light within the diamond.

In summary, the higher the density of a material, the stronger the interaction between light and the material's particles, leading to a decrease in the speed of light and a higher refractive index.