How can we get the white light back when two prism are placed in succession

If we put a single prism in the path of light, we get the spectrum of light. But if we place another inverted prism in front of the former prism, the spectrum merges to form back the white light.
 
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It depends on what you want to achieve. The first prism takes a beam of white light and creates a fan of colored beams at different angles. Putting a second prism in the same way round doubles the angular separation and width of the fan. That's useful for some purposes.

Conversely, inverting the second prism undoes the change in angle, so the different colors are going in the same direction again, but, because the beams travelled for a bit between the prism while they were still at different angles, they're now separated sideways and running in parallel. That's also useful for some purposes.

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