I am doing a final year project, my Prof gave me 2 prism. He wants me to do so: A white laser is shot on a prism producing a visible light spectrum, use another prism to converge the spectrum back into a laser. For me, it is theoretically impossible and I tried to convince him that it doesn't work as prism can only make the light more disperse. But he insisted that by principle of reversibility, there should be a way to convert the spectrum back to a laser. Is he really correct? I want to disprove his statement but I cannot figure out a concrete proof. I don't know what to do now.