Question

 What will happen if another similar glass prism is placed upside down behind the first prism?

Answer 1

If a second, similar glass prism is placed upside down behind the first prism, recombination of seven colours takes place and a patch of ordinary white light is obtained on a screen placed behind the second prism. The recombination of colours took place because the second prism was placed upside down, because of which the refraction produced by the second prism was equal and opposite to the refraction produced by the first prism.