If light is to undergo dispersion, there should always be deviation for light rays after refraction. in a glass prism this takes place, hence light is dispersed by the prism. But, in the case of a glass slab,there is no deviation for light rays, as incident and emergent rays are parallel. hence, light cannot be dispersed by glass slab.
I dont think thats correct
I got the ans from my teacher just now
Its coz glass slab is like a prism and an inverted one put together