Question

Why there is no spectrum from glass slab?

Answer 1

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. 

​Now if we try to observe the spectrum due to glass slab, the convex lens of our eyes converge the parallel beam into white light once again. But we can observe the dispersion effect properly with a prism.