why does a glass slab not disperse white light ?

The light strikes on one surface of the prism undergoes refraction and passes through the glass of the prism, and when emerges out, it refracts again. This causes the dispersion of the light and splits the light into its constituent colours. The degree of bending depends on the angle at which the ray of light enters and leaves the faces of the prism. The faces of the prism are not parallel to each other. For a glass slab, the opposite faces of the slab are parallel. So only a slight deviation occurs.

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 A nice question which takes us to the basics of dispersion.
Suppose that a parallel-sided glass slab is placed in a beam of white light. Dispersion takes place inside the slab, but, since the rays which emerge from the slab all run parallel to one another, the dispersed colours recombine to form white light again, and no dispersion is observed except at the very edges of the beam. 
It follows that the dispersion of white light through a parallel-sided glass slab is not generally a noticeable effect.

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