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.

A measure of the extent to which a substance slows down light waves passing through it. The index of refraction of a substance is equal to the ratio of the velocity of light in a vacuum to its speed in that substance. Its value determines the extent to which light is refracted when entering or leaving the substance.

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.