could you say to me if white light in a prism with refraction => if we recompose the spectrum we will have white light again ? or what else ? since brown and magenta are not on the spectrum. What is the "frequency" of a natural white light and the "frequency" of a recomposed "white" light from a spectrum?
Recombination of white light might or might not happen, this happens on the actual setup that you are using. If you are just using two of the same type of prisms, light will not recombine.
There are several ways to recombine white light, you can have three laser pointers, with two prisms where the height is more then the distance between the upper and lower stacked pointers. In another setup, you need a lens of focal length f at a distance of 2f from the first prisms.