Answer 1

Denser medium are those in which speed of light gets reduced when light travels through that medium.
when light travels through water or glass it's speed get reduced hence they are optically denser medium.

WATER is optically DENSER as the speed of light is less in water than in air.
2.25 × 10⁸ ms^-1   <   3 × 10⁸ ms^-1
speed of light in Water < speed of light in Air.