Question

Why there is no dispersion of logli through a rectangular glass slab?

Answer 1

Dispersion occurs when light is passed through a prism. The dispersion is caused due to the the difference in the angle of deviation for different colours. In the case of a glass slab, the opposite sides are parallel and therefore different colors emerge parallel to each other and are seen simultaneously. Therefore, dispersion doesn’t occur in a glass slab.
In a prism, the dispersion occurring at one face in enhanced by the refraction at the other face, but in a glass slab the dispersion occurring at one face is neutralized by refraction at the other face.
Also,

The light ray incident on a glass at A comes from the opposite at B. Inside the glass slab, the seven constituent colors travel the same distance and emerge through B.

Whereas in the case of Prism, the distance travelled by different colors are diffrrent because of the prism shape (of the order of few wavelength).