Question

Why does dispersion takes place when light passes through a prism and not through a glass slab?

Answer 1

this is because of the shape of the prism .as the light passes through the prism it will deviate away from the normal .
 in the case of the glass slab it is plane and there is not a perticular shape and the medium to the light to pass through the slab .we can also see that the curved end of the glass make a small dispersion and convert into 7 colors 
as in the case of the prism there is a hard medium of glass molecule make the dispersion easier 
     hope it will help u

Answer 2

The reason of the dispersion is that a transparent medium (like a glass prism) bends different colours of light by different amounts. The deviation is maximum for voilet and minimum for red. Therefore, when white light passes through a prism, the different colours present in it get deviated by different amounts. Hence, they separate out on emerging from the prism and form a spectrum.