DISPERSION OF LIGHT
Dispersion is the splitting up of white light into seven colors on passing through a transparent medium like a glass prism. When a white light beam is passed through a prism, a band of seven colors are formed is known as spectrum of white light. The seven colors in the spectrum are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red).
The formation of spectrum of seven colors shows that white light is a mixture of seven colors. The effect of transparent medium (like glass prism, water droplet) is only to separate the seven colors of white light. The dispersion of light occurs because the angle of refraction (or angle of bending) of lights of different colors is different when passing through the transparent medium (glass prism).
So when white light consisting of seven colors fall on a transparent medium (prism) each color in it is reflected or deviated by a different angle, with the result that seven colors are spread out to form a spectrum. The red color is deviated least so it forms the upper part of the spectrum. On the other hand, violet color is deviated the maximum, so violet appears at the bottom of the spectrum.