Fluorescence: What It is and How It Works

Fluorescence occurs naturally in geology and in the oceans. Yet it has many practical applications in everyday life. So what is fluorescence? And how does it work?

By: Joan Whetzel

Fluorescence occurs naturally in geology (rocks) and aquatic settings (fish, dee sea animals, algae). It come about when the fluorescent object absorbs light and reemits it, usually as a different color. Fluorescents has a number of applications in science and in everyday life.

 

What Is Fluorescence?

In fluorescence, an object like a fluorescent rock, is excited by electromagnetic radiation EMF), then reemits visible light when the  EMF source is removed.  Physicist George Stokes discovered the phenomenon in 1882 and developed the Stokes Law of Fluorescence. This law states that the re-emitted fluorescent  always has a greater wavelength and lower frequency of photons than the photons in the EMF light that triggers the response, which have a short wavelength and higher frequency. This reaction is sometimes called Stokes’ Fluorescence, and the difference in wavelength or frequency between the EMF photons and the fluorescence photons is called Stokes’ Shift. The main types of EMF light used to excite the fluorescing photons include visible light, ultraviolet (UV) light, infrared light, x-rays and radio waves.

 

It’s a two step process. First, a small portion of the EMF light must be absorbed. Then, once the EMF light source is removed, the much dimmer fluorescent light is emitted (as a form of luminescence), usually in a different color, depending on the minerals present. In daylight conditions, fluorescence cannot be seen because the sunlight is so much stronger than the fluorescence that it is pretty much washed out.

 

Minerals sometimes contain impurities, referred to as “activators” which create the color within the fluorescence. The type of mineral activator necessary for fluorescence to occur, depends on the base mineral. Some impurities (e.g. iron, copper), known as quenchers, inhibit fluorescence even with a mineral activator is present. To prevent quenching (dousing) the fluorescent re-emissions in some cases, two things are important to consider. First, the sample must not be flooded with too much light from the activator and, second, the sample must not contain iron or copper.

 

Fluorescence in Nature

Stokes invented the term “fluorescence,” naming the process after the mineral fluorite, which is intensely fluorescent. Currently, there are over 3600 species of minerals known to have fluorescence capabilities, that re-emit light in different colors, depending on the mineral activators. Some of the fluorescing minerals include:

·         Calcite which fluoresces in most colors. Red and pink fluorescent calcites are usually activated by a combination of lead and manganese, while green calcite fluoresces due to traces of uranyl ion.

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