I was shown this trick at Sarah and Matt's wedding reception. You can't see anything on this paper.
What's going on here ties in with what I was telling my Applied Science class. This is how reflection works. A photon of light enters an atom, as shown below. The energy helps an electron to go several energy levels.
When the electron falls back, it loses energy again by firing out a photon, which is what we see.
The electron arrangement in the ink is clearly such that ordinary light does not have enough photon energy to allow emission of photons that we can see. Ultra-violet light has more energy in its photons. However, surely the re-emitted photons would be the same invisible high frequency. What must be happening is that the electron doesn't fall straight down but jumps from one energy level to another, releasing some photons that can be seen by the eye.
