Why does dispersion of light takes place only in prism and not any other transparent object?

A light ray is refracted (bent) when it passes from one medium to another at an angle and its speed changes. At the interface, it is bent in one direction if the material it enters is denser (when light slows down) and in the OTHER direction if the material is less dense (when light speeds up).
The basic reason behind the dispersion of white light into seven different colours is because all the seven rays of light of different colours travel at different speeds through the glass prism. The degree of refraction also can be said as the bending of each light ray depends upon their individual speed while passing through the prism. All the different colours of light ray travel at different speeds in glass and so the angle of bending or angle of refraction of each light ray is different.

The dispersion occurs in prism but not in glass slab because of the geometric design. In a slab, the opposite sides are parallel to each other whereas, in the case of the prism, the sides are not parallel to each other. The dispersion of light does occur in a rectangular glass slab during the first interface of air and glass slab. They travel a very small distance in the glass slab to meet the second interface.