# Is it possible to implement the reversed dispersion of a white light beam and how?

The dispersion of white light beam while passing through a triangular prism is well known. Considering the reversibility of optical path, it should be possible to reverse the experiment.

1. How can one implement the reverse experiment of lights' dispersion in experiment?

2. What are the necessary and sufficient conditions of the incident light beam(s) in order to obtain a white light beam after passing through a dispersive prism? For example, can one remove some of light beams with specific wavelengths or adjust their intensity but still obtains white light beam?

It is indeed possible. This was a famous experiment by Isaac Newton (published in 1672). Place a lens of focal length $f$ a distance $2f$ from the first prism. Add a second identical prism $2f$ past the lens and rotate it round until white light emerges.