Dispersion of light

Dispersion of light is occurred through a prism but not through a glass slab whereas changes in medium is in same way i.e. from optically rarer medium to optically denser and again from optically denser to optically rarer medium.

In prism light ray deviated in the same direction while entering or, leaving the prism. That’s why it creates a angle of deviation and dispersion is occurred.

Whaereas in glass slab light rays deviate in opposite direction by same angle while entering or, leaving the glass slab. That’s why it doesn’t create angle of deviation or, resultant angle of deviation is zero. Without angle of deviation dispersion of light is not possible.

That’s why no dispersion of light in glass slab. As explain by figure in sideways.