Dispersion is the splitting up of white light (or a composite light) into its constituent colours.
Dispersion occurs when light is passed through a prism. The dispersion is caused due to the the difference in the angle of deviation for different colours.
In the case of a glass slab, the opposite sides are parallel and therefore different colors emerge parallel to each other and are seen simultaneously. Therefore, dispersion doesn’t occur in a glass slab.
In a prism, the dispersion occurring at one face in enhanced by the refraction at the other face, but in a glass slab the dispersion occurring at one face is neutralized by refraction at the other face.
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