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In other fields, the dispersion can be a nuisance: in camera lenses it leads to chromatic aberration and in optical cables chromatic dispersion limits the maximum bandwidth of the fiber (though this effect is not as strong as modal dispersion which is caused by longer paths of reflected rays as opposed to those that travel straight through along the core. 

Refraction:

It’s the change in direction of a wave due to a change in its medium. It is essentially a surface phenomenon. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°.

 Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth. With light, the speed in transparent medium is lower than in vacuum since light can travel in vacuum and doesn’t need a medium to travel. We characterize the transparent medium by its Index of Refraction, defined as the ratio of the speed of light c in vacuum to the speed on light in the medium:

Explanation:

 Refraction of [light|http://en.wikipedia.org/wiki/Light] is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when [sound waves|http://en.wikipedia.org/wiki/Sound_wave] pass from one medium into another or when water waves move into water of a different depth\[1\]. With light, the speed in transparent medium is lower than in vacuum since light can travel in vacuum and doesn’t need a medium to travel. We characterize the transparent medium by its *Index of Refraction*, defined as the ratio of the speed of light c in vacuum to the speed on light in the medium:
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n=c/v    (index of refraction)      eq….1  \[2\]

n is always 1 or greater depending upon the medium. The index of refraction for some familiar substances:

• Vacuum, defined as 1
• Air, approximately 1.0008
• Water, 1.33
• Glass, 1.5
• Diamond, 2.2

The index of refraction is a way of expressing how optically dense a medium is. The actual index of refraction (other than in a vacuum) depends on the incoming wavelength. Different wavelengths have slightly different speeds in (non-vacuum) mediums. For example, red slows down by a certain amount, but violet slows down by a slightly lower amount - meaning that red light goes through a material (glass, for example) a bit faster than violet light. Red light exits first\[4\].
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When wave enters from one medium to another, the wave speed changes but the frequency of the wave remains the same, meaning that the speed of wave depend upon the change in wavelength according to the formula    v=fλ                  eq….2.

After comparing equation 1 and 2 we get λ=c/nf and since c and f don’t change, the wavelength is inversely proportional to n.

 Refraction is described by Snell's law, which states that for a given pair of media and a wave with a single frequency, the ratio of the sines of the angle of incidence θ₁ and angle of refraction θ₂ is equivalent to the ratio of phase velocities (v₁ / v₂) in the two media, or equivalently, to the opposite ratio of the indices of refraction (n₂ / n₁):

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 Refraction is described by [Snell's law|http://en.wikipedia.org/wiki/Snell%27s_law], which states that for a given pair of media and a wave with a single frequency, the ratio of the sines of the [angle of incidence|http://en.wikipedia.org/wiki/Angle_of_incidence] _θ{_}{_}{~}1{~}_ and [angle of refraction|http://en.wikipedia.org/wiki/Angle_of_refraction] _θ{_}{_}{~}2{~}_ is equivalent to the ratio of phase velocities (_v{_}{_}{~}1{~}{_}_ / v{_}{_}{~}2{~}_) in the two media, or equivalently, to the opposite ratio of the indices of refraction (_n{_}{_}{~}2{~}{_}_ / n{_}{_}{~}1{~}_)\[1\]:
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!s.png|border=1!\[1\]

\\  !p.png|border=1!\[1\]
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Common Example:

Going from a medium of lower refractive index, to a medium of higher refractive index, the beam bends toward the normal. In opposite case, when light travels from a higher refractive index to lower refractive index, the beam bends away from the normal.
\\  !brokpen.jpg|border=1!\[3\]

Common Example:

Going from a medium of lower refractive index, to a medium of higher refractive index, the beam bends toward the normal. In opposite case, when light travels from a higher refractive index to lower refractive index, the beam bends away from the normal.
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Further Reading/Watching

1. The speed of light in a glass

References:

1.http://en.wikipedia.org/wiki/Refraction^{Image Added}

2.Essential University Physics/Richard Wolfson

3.http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr2.html#c1^{Image Added}

4.http://howthingsworkclass.blogspot.fi/2012/10/reflection-and-refration-reflection.html^{Image Added}