The density of a substance will increase the refractive index of the substance (meaning light will travel slower through the substance).

Does density increase refractive index?

In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relationship between the refractive index and the density for all silicate and borosilicate glasses.

How does density affect refraction?

The angle and wavelength at which the light enters a substance and the density of that substance determine how much the light is refracted. ... When light passes from a more dense to a less dense substance, (for example passing from water into air), the light is refracted (or bent) away from the normal.