Fluorescence

Fluorescence is a luminescence, i.e. optical phenomenon in cold bodies, in which a molecule absorbs a high-energy photon, and re-emits it as a lower-energy (longer-wavelength) photon. The energy difference between the absorbed and emitted photons ends up as molecular vibrations (heat). Usually the absorbed photon is in the ultraviolet, and the emitted light (luminescence) is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. Fluorescence is named after the mineral fluorite (calcium fluoride), which exhibits this phenomenon.

Equation

:S_1 o S_2 + h u

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This means that the system starts in state S_1, and after the fluorescent emission of a photon with energy h u, it is in state S_2 where:

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h = Planck's constant and

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u = frequency of the fluorescing light

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