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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. 

 
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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 Wiki MarkupThe refractive index of materials varies with the wavelength (and&nbsp;frequency) of light. This is called dispersion and causes&nbsp;prisms&nbsp;to divide white light into its constituent spectral&nbsp;colors, and explains how&nbsp;rainbows&nbsp;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&nbsp;optics,&nbsp;*dispersion*&nbsp;is the phenomenon in which the&nbsp;phase velocity&nbsp;of a wave depends on its frequency,^\[1\]^&nbsp;or alternatively when the&nbsp;group velocity&nbsp;depends on the frequency. The *refractive index* of materials varies with the wavelength (and&nbsp;frequency) of light. Media having such a property are termed _dispersive media_. Dispersion is sometimes called&nbsp;*{_}chromatic{_}{*}*&nbsp;dispersion*&nbsp;to emphasize its wavelength-dependent nature, or&nbsp;*group-velocity dispersion*&nbsp;(*GVD*) to emphasize the role of the group velocity. Dispersion is most often described for&nbsp;light&nbsp;waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g., a&nbsp;[<span style="color: #0b0080">waveguide</span>|http://en.wikipedia.org/wiki/Waveguide]), such as&nbsp;sound&nbsp;waves \\ !worddav6ae8caedb03f53d95f48fad04e76b693.png|height=174,width=277! Fig 01: A&nbsp;dispersive prism, material dispersion (a&nbsp;[<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]&nbsp;at different angles, splitting white light into a&nbsp;[<span style="color: #0b0080">rainbow</span>|http://en.wikipedia.org/wiki/Rainbow].

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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