Question

why do glass slab doesn't show dispersion of light and prism show it?

Answer 1

The light strikes on one surface of the prism undergoes refraction and passes through the glass of the prism, and when emerges out, it refracts again. The faces of the prism are not parallel to each other. This causes the dispersion of the light and splits the light into its constituent colours. The degree of bending depends on the angle at which the ray of light enters and leaves the faces of the prism.