Dispersion
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The refractive index of materials varies with the wavelength (and frequency) of light. This is called dispersion and causes prisms to divide white light into its constituent spectral colors, and explains how rainbows are formed. As the refractive index varies with wavelength, according to Snell's law, so will the refraction angle as light goes from one material to another. This makes different colors go in different directions.
In optics, {*}dispersion{*} is the phenomenon in which the phase velocity of a wave depends on its frequency,{^}\[1\]{^} or alternatively when the group velocity depends on the frequency. The *refractive index* of materials varies with the wavelength (and frequency) of light.
Media having such a property are termed _dispersive media{_}. Dispersion is sometimes called {*}{_}chromatic{_} dispersion{*} to emphasize its wavelength-dependent nature, or {*}group-velocity dispersion{*} ({*}GVD{*}) to emphasize the role of the group velocity.
Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g., a [<span style="color: #0b0080">waveguide</span>|http://en.wikipedia.org/wiki/Waveguide]), such as sound waves
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Fig 01: A dispersive prism, material dispersion (a [<span style="color: #0b0080">wavelength</span>|http://en.wikipedia.org/wiki/Wavelength]-dependent [<span style="color: #0b0080">refractive index</span>|http://en.wikipedia.org/wiki/Refractive_index]) causes different colors to [<span style="color: #0b0080">refract</span>|http://en.wikipedia.org/wiki/Refraction] at different angles, splitting white light into a [<span style="color: #0b0080">rainbow</span>|http://en.wikipedia.org/wiki/Rainbow].
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Refractive index
In optics the refractive index (or index of refraction) n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium. Its most elementary occurrence (and historically the first one) is in Snell's law of refraction, n1sinθ1= n2sinθ2, where θ1 and θ2 are the angles of incidence of a ray crossing the interface between two media with refractive indices n1 and n2.
Simply, it is the Specific property of medium to propagate or refract light or radiation.
Chromatic Aberration
In Optics, chromatic aberration (CA, also called achromatism or chromatic distortion) is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light (the dispersion of the lens). The refractive index decreases with increasing wavelength.
In simple, it is condition in which the lens fails to focus all colors at conversing point due to Dispersion Of the lens.
Fig 02: Showing effect of aberration on photography and correction with use of wide angle Lens
Mathematical analysis of Dispersion of light
A material's dispersion is measured by its Abbe number, V, with low Abbe numbers corresponding to strong dispersion. For optics in the visual range the amount of dispersion of a lens material is often quantified by the Abbenumber . 