Figure 1.

Diagram of ground and excited states of electrons. Incident photons of the right energy can cause electrons in their ground state to jump to permitted excited states. In excited states, some of this energy can be transferred to vibrational states, causing electrons to decay to lower energy states. Photons emitted during the energy transfer process as electrons decay to their original ground state are of lower energy (and hence longer wavelength) than the incident light. If the emitted light is in the visual range, fluorescence is observed.