In his 1852 paper on the "Refrangibility" (wavelength change) of light,George Gabriel Stokes described the ability of fluorspar and uranium glass to change invisible light beyond the violet end of the visible spectrum into blue light. He named this phenomenon fluorescence : "I am almost inclined to coin a word, and call the appearance fluorescence, from fluor-spar [i.e., fluorite], as the analogous term opalescence is derived from the name of a mineral." The name was derived from the mineral fluorite (calcium difluoride), some examples of which contain traces of divalent europium, which serves as the fluorescent activator to emit blue light. In a key experiment he used a prism to isolate ultraviolet radiation from sunlight and observed blue light emitted by an ethanol solution of quinine exposed by it.
Physical principles 
Fluorescence occurs when an orbital electron of a molecule, atom or nanostructure relaxes to its ground state by emitting a photon of light after being excitedto a higher quantum state by some type of energy:
Fluorescence (emission): [pic]
here [pic] is a generic term for photon energy with h = Planck's constant and [pic] = frequency of light. (The specific frequencies of exciting and emitted light are dependent on the particular system.)
State S0 is called the ground state of the fluorophore (fluorescent molecule) and S1 is its first (electronically) excited state.
A molecule, S1, can relax by various competing pathways. It can undergo 'non-radiative relaxation' in which the excitation energy is dissipated as heat(vibrations) to the solvent. Excited organic molecules can also relax via conversion to a triplet state, which may subsequently relax via phosphorescence or by a secondary non-radiative relaxation step.
Relaxation of an S1 state can also occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen (O2) is an extremely efficient quencher of fluorescence just because of its unusual triplet ground state.
Molecules that are excited through light absorption or via a different process (e.g. as the product of a reaction) can transfer energy to a second 'sensitized' molecule, which is converted to its excited state and can then fluoresce. This process is used in lightsticks to produce different colors.
Quantum yield 
The fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed.
The maximum fluorescence quantum yield is 1.0 (100%); every photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence, is by the rate of excited state decay:
where [pic] is the rate of spontaneous emission of radiation and
is the sum of all rates of excited state decay. Other rates of excited state decay are caused by mechanisms other than photon emission and are, therefore, often called "non-radiative rates", which can include: dynamic collisional quenching, near-field dipole-dipole interaction (or resonance energy transfer),...