Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation.
The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible, and the emitted light is in the visible region.
George Gabriel Stokes named the phenomenon fluorescence in 1852. He chose the name "to denote the general appearance of a solution of sulphate of quinine and similar media". 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.
Fluorescence has many practical applications, including mineralogy, gemology, chemical sensors, fluorescent labelling, dyes, biological detectors, and, most commonly, fluorescent lamps.
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 excited to a higher quantum state by some type of energy:
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