Refraction is when a light beam changes direction after entering a new medium. This is why objects in the water are seen at a different place from where they actually are; the light has been “bent” when exiting the water. The image below shows light being refracted through plastic.
This happens because media generally have different indices of refraction. The index of refraction is defined as the ratio of the speed of light inside the medium to the speed of light in vacuum:
where nis the index of refraction, cis the speed of light in vacuum, and vis the speed of light in the medium. The angle of refraction is described by Snell’s law:
where is the angle to the surface normal, subscript 1 indicates quantities related to the first medium, and subscript 2 indicates the second medium.
