Dispersion

Dispersion (light)

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.
    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.

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

Source:Wikipedia Chromatic aberration

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