Why do you not see a spectrum of colours when light passes through a flat pane of glass ?

Is there a spectrum observed when white light is passed through a thick rectangular glass slab ?

Why white light splits into different colours when it passes through a prism ?

Answer the following questions What is a spectrum ? why do we get a spectrum of seven colours when while light is dispersed by a prism ?

A glass prism produces a spectrum when white light passes through it but a glass slab does not do so. Explain.