ultraviolet

If you visit an archive with scrolls, illuminated manuscripts, or old books on display, you might notice a particular kind of round, yellowish, or orange-tinted stain on some of the paper artifacts. This color effect is called foxing, and paper so affected is said to be “foxed.”

Have you ever wondered why fluorescent colors like the ones you see in highlighers or clothing dyes seem so much brighter than other colors? It's because they seem to reflect more light than they receive.