Refraction of Light by Water

Light entering or exting a water surface is bent by refraction. The index of refracton for water is 4/3, implying that light travels 3/4 as fast in water as it does in vacuum.

Refraction at the water surface gives the "broken pencil" effect shown above. Submerged objects always appear to be shallower than they are because the light from them changes angle at the surface, bending downward toward the water.