Scientists in France have created paper that can carry secret messages. In visible light, the paper is indistinguishable from regular paper and users can read, write or erase messages using three different wavelengths of UV light.
The functionalised paper, made by François-Xavier Felpin from the University of Nantes, and colleagues, contains coumarin molecules attached to the paper’s cellulose fibres. Exposing the paper to UV light with a wavelength of 340nm causes coumarin to react and create cyclobutane dimers. These dimers are invisible under visible light, but fluoresce under a UV lamp.