Light guidance by optical devices
Geometry of the light rays: from theory towards applications

Light guides are optical devices that channel light rays along a regular curve in the direction of light propagation. They can be built around discrete elements such as diopters or mirrors, or be continuous, using graded-index materials. The most common technique is to adjust two diopters around a rectilinear axis to design lenses and mirrors for optical instruments. Well-known light guides are those that preferably use graded-index materials, such as optical fibers and graded-index lenses.

These systems imply that Descartes' law is described in fixed reference frames (§ 4.2