Question

will a glass slab also form a spectrum like a Prism .explain

Answer 1

heya
No, Prism is made in such a way to form a spectrum.. Glass Slab will not be able to form Spectrum.
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. Thus the emergent beam is a parallel beam in case of a glass slab, while it is divergent in case of a prism

Answer 2

a prism can easily produce a spectrum but glass slab after refraction at the two parallel faces of glass slab a ray  of light emerges to the direction  parallel to the direction of incidence ray on white slab,rays of all color emerge in same direction of incidence white light and thus no spectrum is produced