The bending of a beam of radiation as it passes from one medium onto another. For a light ray the amount by which it is bent depends on the angle of the incident ray and on the refractive indices of the two media, the exact dependence being given by Snell's law .
Refraction is caused by the difference in the speed of the radiation in the two media.
The ratio of the speed of light in the two media is known as the refractive index . If the first medium is a vacuum, the ratio is known as the absolute refractive index of the second medium. The Macmillan Encyclopedia 2001 2. Snell's law Oxford Paperback Encyclopedia 3. Snell's law A Dictionary of Science, Oxford University Press 4.