Question

Why does the dispersion of white light doesnt takes place in rectangular glass slab?

Answer 1

Dispersion of light occur in glass slab. Consider a thin beam of white light which is being refracted by a rectangular glass slab. The colors do disperse at the first interface of glass and air. But, after being refracted through the second interface, all colors start moving parallel to each other.

A prism has not parallel surfaces so it disperses the white light more accurately.