Fluorescence

Atoms that absorb u.v. light and become excited are able to emit visible light when they de-excite.This is because an atom in the excited state can de-excite directly or indirectly to the ground state.The atoms emit photons of less energy by de-excitation in stages.A mercury atom in the ground state can absorb a photon of 6.7eV and then de-excite to 4.9eV emitting a photon of 1.8eV and then a photon of 4.9eV to return to the ground state.When the ultraviolet source is removed the atoms stop glowing.