What is meant by dispersion of white light? draw a ray diagram to show dispersion of white light by a glass prism? why do we get different colours?

Please refer to the diagram given above (left prism only.)
Dispersion of light occurs in prism because light rays deviate while coming out of a prism by an angle. This is called the 
angle of deviation. Now angle of deviation depends upon the refractive index of the medium of which the prism is composed. Now since different colors in the white light have got different refractive index for a given material of the prism. Thus each color deviates by different angle from the other. Thus, these constituent colors separate out.

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The phenomenon of splitting up of a composite light into its constituent colours is known as dispersion. For example, when sunlight is passed through a prism, it undergoes dispersion and splits into its seven constituent colours- violet, indigo, blue, green, yellow, orange and red.

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