CBSE Class 10 Answered

it happens in the glass slab too. But since the two faces of the slab are parallel, all the light ends up going in the same direction again when it emerges, while the prism further spreads the angles. If you had a realllly thin beam of light and shined it on a thick slab of glass, then looked at the emerging beam very closely, you'd see a little bit of rainbow effect. Normally, though, the amount of spread that happens in a glass slab isn't enough to notice. The prism takes advantage of the fact that the different colors leave at different angles and keep spreading out until you can tell the difference