Gradient-index optics

[2] 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.

[3] This effect also allows for observation of electromagnetic signals from satellites after they have descended below the horizon, as in radio occultation measurements.

[4] The flat surface also allows a GRIN lens to be easily optically aligned to a fiber, to produce collimated output, making it applicable for endoscopy as well as for in vivo calcium imaging and optogenetic stimulation in brain.

A number of different materials have been used for GRIN lenses including optical glasses, plastics, germanium, zinc selenide, and sodium chloride.

[7] An axial gradient lens has been used to concentrate sunlight onto solar cells, capturing as much as 90% of incident light when the sun is not at an optimal angle.

[8] GRIN lenses are made by several techniques: In 1854, J C Maxwell suggested a lens whose refractive index distribution would allow for every region of space to be sharply imaged.

In 1905, R. W. Wood used a dipping technique creating a gelatin cylinder with a refractive index gradient that varied symmetrically with the radial distance from the axis.