Europium-Doped Glass
A germanate glass sample doped with europium, one of the rarest of the rare-earth elements. When illuminated with invisible ultraviolet light, the europium atoms absorb this radiation and emit red light—making the invisible visible. One also sees patterns emerging from internal reflection at the edges.
—Susanne Viezens, Max Planck Insitute for the Science of Light, Erlangen, Germany