Which law is used to determine the change of a ray's direction when passing between two mediums?

The determination of how a ray's direction changes when it passes from one medium to another is governed by Snell's Law. This law describes the relationship between the angles of incidence and refraction as light passes through different media with varying indices of refraction. Snell's Law can be mathematically expressed as the ratio of the sines of the angles being equal to the ratio of the indices of refraction of the two media involved.

In practical terms, this means that when light enters a medium where it travels faster or slower, it bends at a certain angle. For instance, when light moves from air into glass, it slows down and changes direction, which can be predicted using Snell's Law. This principle is fundamental in optics and is critical in applications involving lenses and prisms, enhancing our understanding of how light behaves in different environments.