Question

How does recombination of spectrum colours occur?

Answer 1

When white light is passed through the first prism, it disperses the white light into seven colours. The second identical glass prism kept upside down recombines these rays into the white light. This recombination occurs due to the reverse refraction produced by the second prism kept upside down with its refracting sides parallel to each other. On the other hand if the second prism is kept just like the first prism, then no recombination of dispersed light occurs.