Why does the light splits in prism but not in glass slab??

Dispersion of light as a special case of refraction too. now we now that refraction is bending of light at the interface of two different media. One important feature of the glass slab is that the light gets refracted twice and the two faces of glass slab are parallel to each other . see the figure below:

In case of dispersion, the white light gets split into its constituent colours. Since according to Cauchy’s formula, refractive index of a material depends on wavelength of light,therefore, refractive index of the material is different for different colours/wavelengths.

Now see the figure below:

 As the refractive index of the prism  is different for different colours, therefore , different colours deviate through different angles on passing through the prism as shown in the figure above. In this case we should note that the opposite faces of the prism are inclined to each other and not parallel.
​Regards