Question

Why does a ray of light passing through a glass slab not show dispersion?

Answer 1

If the two interface surfaces are parallel to each other, as in a 'slab' of glass, all of the bending (and dispersion) that takes place at the first interfaces is exactly reversed at the second surface.

Answer 2

the light rays passing through glass slab are parallel since the surfaces of the slab are also parallel. all the refracted wavelengths emerge parallel to the incident ray direction. hence the light ray do not show dispersion when passing through glass slab. but in case of some other materials like prism, the surfaces are not parallel hence it shows dispersion