Question

why dispersion doesnt occur in glass slab

Answer 1

this is due to all refracted ray emerge parallel to incident ray and will not diverge like the prism

Answer 2

Answer:

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. ... They travel a very small distance in the glass slab to meet the second interface