Question

Why does dispersion not occur in a glass slab?

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, 'undoing' the effect of the first interface; so although the emerging ray of light is displaced slightly from the entering ...

ans
A glass slab produces dispersion but the resulting rainbow like spectrum effect cannot be seen. This is due to the fact that all the refracted wavelengths emerge parallel to their incident direction. ... But we can observe the dispersion effect properly with a prism.