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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.
Source : Wikipedia dispersion
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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
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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 . 
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1) Aggarwala K, Kruger PB, Mathews S. et al Spectral Bandwidth and Ocular Accommodation. Journal of the Optical Society of America, 1995, 12, p 450
2) Isaac Newton: adventurer in thought, by Alfred Rupert Hall, page 67