Refraction

The change in direction of a ray of light when it passes through one media to another with differing optical densities.

Changes in the direction of energy propagation as a result of density changes within the propagating medium. In weather terms, this is important on determining how a radar beam reacts in the atmosphere.

The bending of light as it passes from one medium to another.

Retardation, and--in the general case--redirection, of a wavefront passing through (a) a boundary between two dissimilar media or (b) a medium having a refractive index that is a continuous function of position, e.g., a graded-index optical fiber. Note: For two media of different refractive indices, the angle of refraction is closely approximated by Snell's Law.