Editing Gradient-index optics (section)

== In nature ==
The [[Lens (anatomy)|lens]] of the [[eye]] is the most obvious example of gradient-index optics in nature. In the [[human eye]], the [[refractive index]] of the lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens.<ref>{{Cite book |title=Optics |last=Hecht |first=Eugene |date=1987 |publisher=Addison-Wesley |last2=Zając |first2=Alfred |isbn=978-0201116090 |edition=2nd |location=Reading, Mass. |pages=178 |oclc=13761389}}</ref> This allows the eye to image with good resolution and low aberration at both short and long distances.<ref>Shirk J S, Sandrock M, Scribner D, Fleet E, Stroman R, Baer E, Hiltner A. (2006) ''NRL Review'' pp 53–61</ref>

Another example of gradient index optics in nature is the common [[mirage]] of a pool of water appearing on a road on a hot day. The pool is actually an image of the sky, apparently located on the road since light rays are being [[refracted]] (bent) from their normal straight path. This is due to the variation of refractive index between the hot, less dense air at the surface of the road, and the denser cool air above it. The variation in temperature (and thus density) of the air causes a gradient in its refractive index, causing it to increase with height.<ref name=":0">Tsiboulia, A B (2003). "Gradient Index (GRIN) Lenses". In Ronald G. Driggers. ''Encyclopedia of Optical Engineering'', Volume 1. New York, NY: Marcel Dekker. 675-683. {{ISBN|9780824742508}}.</ref> This index gradient causes refraction of light rays (at a shallow angle to the road) from the sky, bending them into the eye of the viewer, with their apparent location being the road's surface.

The Earth's atmosphere acts as a GRIN lens, allowing observers to see the sun for a few minutes after it is actually below the horizon, and observers can also view stars that are below the horizon.<ref name=":0" /> This effect also allows for observation of electromagnetic signals from satellites after they have descended below the horizon, as in [[radio occultation]] measurements.