Refractive index of glass: 1.52 to 1.9

Refractive index of water: 1.33

Refractive index of diamond: 2.42

A substance having higher refractive index is optically denser then the substance of lower refractive index and light will travel slowest in the most optically dense substance i.e. having highest refractive index.

(a) Light travels slowest in the diamond having highest optical density.

(b) Light travels fastest in the water having lowest optical density.