Refraction is the change in a wave's direction when it encounters a change in its transmission medium. The process of refraction allows eyes and lenses to form images. When the wave's medium changes, the velocity of the wave changes, but its frequency remains the same.
When light passes from a fast medium to a slower medium, refraction causes the light wave to bend. The amount of bending that takes place depends on the indices of refraction for the two different media. To calculate the index of refraction, the velocity of light in a vacuum is divided by the velocity of light in a given medium. Snell's Law describes the relationship between two different indices of refraction.