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Dispersion

A single ray of light consist of seven different color with varying wavelength. When a ray of light travels from one transparent medium to another(e.g from air to glass) then it splits into its constituent color (Red,Orange,Yellow,Green,Blue,Indigo,Violet). This phenomenon by which a ray of light  slits into its constituent color when it is passed through a transparent medium is called dispersion. Refraction is the cause and dispersion is the result. 

 

Fig 01: A dispersive prism, material dispersion (a wavelength-dependent refractive index) causes different colors to refract at different angles, splitting white light into a rainbow.

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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 Abbe number  .