Fluorescence is the emission of light by a substance that has previously absorbed light or other electromagnetic radiation. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation (Figure 1). This light emission would cease immediately upon removal of the excitation source. Hence, it is not a persistent phenomenon.
Figure 1. Jablonski diagram of fluorescence. After an electron absorbs a high energy photon, the system is excited electronically and vibrationally. The system relaxes vibrationally and, eventually, fluoresces at a longer wavelength.
The use of fluorescence in life sciences inspired the creation of new techniques and revolutionized existing ones, for instance fluoresecent microscopy, qPCR, flow cytometry and FACS. This was possible due to the discovering of fluorophores, such as the green fluorescent protein.