Question

explain why to prism placed next to each other such that their adjacent faces are parallel behave like a glass slab?

Answer 1

Answer.     It is because a ray of white light falling on the first prism will disperse the light into its seven contituent colours. this band of seven colours, called a spectrum then falls on the parallel adjacent side of the second prism.  This is why two prisms with their adjacent sides parallel behave like a glass slab as a glass slab does not disperse the light but gives it a slight angle of deviation.       

Answer 2

Two prisms placed next to each other such that their adjacent sides are parallel ,behave like a glass slab because a ray of white light falling on the first prism will disperse the light into its seven contituent colours. this band of seven colours, called a spectrum then falls on the parallel adjacent side of the second prism. This causes the spectrum or the band of seven colours to reunite and form the ray of white light. This is why two prisms with their adjacent sides parallel behave like a glass slab as a glass slab does not disperse the light but gives it a slight angle of deviation.