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Fluorescence is an optical phenomenon 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 ending up as molecular vibrations (heat). Usually the absorbed photon is in the ultraviolet, and the emitted light (luminescence) is in the visible range. Fluorescence is named after the mineral fluorspar (calcium fluoride), which exhibits this phenomenon.

There are many natural and synthetic compounds that exhibit fluorescence, and they have a number of applications:

Table of contents
1 Lighting
2 Biochemistry & Medicine
3 Gemmology, Mineralogy and Forensics


The common fluorescent tube relies on fluorescence. Inside the glass tube is a partial vacuum and a small amount of mercury. An electric discharge in the tube causes the mercury atoms to emit light. The emitted light is in the ultraviolet range and is invisible, and also harmful to living organisms, so the tube is lined with a coating of a fluorescent material, called the phosphor, which absorbs the UV and re-emits visible light.

Recently, "white LEDs" (Light Emitting Diodes) have become available which work through a similar process. Typically, the actual light-emitting semiconductor produces light in the blue part of the spectrum, which strikes a phosphor compound deposited on a reflector; the phosphor fluoresces in the orange part of the spectrum, the combination of the two colors producing a net effect of apparently white light.

Biochemistry & Medicine

There is a wide range of applications for fluorescence in this field. Large biological molecules can have a fluorescent chemical group attached by a chemical reaction, and the fluorescence of the attached tag enables very sensitive detection of the molecule. Examples

Gemmology, Mineralogy and Forensics

Gemstones, Minerals, fibers and many other materials which may be encountered in forensics or with relationshp to various collectibles may have a distinctive fluorescence or may fluoresce differently under short wave ultraviolet, long wave ultra violet or X-rays.

Rubies and the Hope Diamond exhibit red fluorescence under short-wave UV light; diamonds also emit light under X ray radiation.