Why dispersion of white light occurs only in prism and not in glass slab? Give suitable reason.

The short answer is that dispersion of light does occur in a rectangular glass slab. It is just that most glass slabs aren’t big enough for this to be noticed. When a narrow beam of white light is refracted at a glass surface, dispersion takes place in such a way as to produce a divergent beam whose components are a spectrum. If this dispersed beam of light passes out of the prism (through the face parallel to the first incident face) the components will be combined to form a beam of white light that has colour distortion at its edges. This colour distortion is commonly known as chromatic aberration and is found in light that passes through all simple lenses including a flat prism when incident light is not perpendicular to the face. If the glass slab was big enough, the path of divergent light would be long enough for a spectrum to be seen when it emerges from the other face. Most glass slabs are not big enough to show this, but all will show the effect of chromatic aberration.

A triangular prism accentuates the dispersion of the beam within the glass by further dispersing the already dispersed mixture of light in the beam within the glass. When the path through the glass is far enough to cause the dispersion to be significantly wider than the width of the incident white beam, a discernible spectrum of light rays diverge from the prism. Most triangular prisms will form a spectrum from a narrow beam of white light.