Editing Refractive index (section)

===Inhomogeneity===
[[File:Grin-lens.png|thumb|alt=Illustration with gradually bending rays of light in a thick slab of glass|A gradient-index lens with a parabolic variation of refractive index ({{mvar|n}}) with radial distance ({{mvar|x}}). The lens focuses light in the same way as a conventional lens.]]

If the refractive index of a medium is not constant but varies gradually with the position, the material is known as a gradient-index (GRIN) medium and is described by [[gradient index optics]].<ref name="Hecht"/>{{rp|273}} Light traveling through such a medium can be bent or focused, and this effect can be exploited to produce [[lens (optics)|lenses]], some [[optical fiber]]s, and other devices. Introducing {{abbr|GRIN|gradient-index}} elements in the design of an optical system can greatly simplify the system, reducing the number of elements by as much as a third while maintaining overall performance.<ref name="Hecht"/>{{rp|276}} The crystalline lens of the human eye is an example of a {{abbr|GRIN|gradient-index}} lens with a refractive index varying from about 1.406 in the inner core to approximately 1.386 at the less dense cortex.<ref name="Hecht"/>{{rp|203}} Some common [[mirage]]s are caused by a spatially varying refractive index of [[Earth's atmosphere|air]].