The dispersion of a light ray does not take place in a glass slab, but it takes place in a prism.....why does this occur?its related to refraction...

The light gets refracted when it changes the medium of
propagation.The refraction or bending of light towards normal  through  surface of
incidense in denser medium and bending away from the normal to the surface when changing
the path  from the denser to less denser medium of propagation is the phenomenon. The
different coloured light with different frequency bend differently  as long as the prism
surfaces on which light is incident and emerging are not parallel. But when the surfaces
are parallel, the effect of dispersion from the incident surface is undone by the other
surface. A glass slab acts in this fashion as it could a join of two prisms which
dispersed light from one surface is again undone by the other surface. But the path
light in such propagation is laterally shifted.