When white is passed through a prism, dispersion of white light into seven constituent colours occurs as the lights of different colours bend through different angles while passing through the prism. This happens due to the difference in the velocities of different colours through the prism. i.e the refractive index of the glass is different for different colours. 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.