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.