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Source : Wikipedia dispersion
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.
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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 . 
References
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