Why dispersion takes place in glass prism but not in the case of rectangular prism??
When a light ray passes through a glass slab, it is actually dispersed while bending, but when it passes a parallel surface all of the bending is reversed, as a result the dispersed radiation recombine to form a white light. Only the emergent ray shows lateral displacement with respect to the incident ray.
When the light ray is incident on a prism it dispersed and bent and this effect is not reversed as the opposite face is not parallel to the first surface it encounters. Hence, seven components are separated by the prism and not a rectangular slab.
In a prism, the refraction of light takes place at the two slant surfaces. The dispersion of white light occurs at the first surface of prism where its constituent colours are deviated through different angles. At the second surface, these splitted colours suffer only refraction and they get further separated. But in a rectangular glass block, the refraction of light takes place at the two parallel surfaces. At the first surface, although the white light splits into its constituent colours on refraction, but these splitted colours on suffering refraction at the second surface emerge out in form of a parallel beam, which give an impression of white light.