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What is Fluorescence?

By Di L.
Updated: May 21, 2024

Fluorescence is an optical phenomenon that is manifested as glowing light. We observe it every day in the energy-saving compact fluorescent light bulbs that are used in many households and some types of glow sticks, but also in nightclubs as well. It is often referred to as “cold light,” because very little heat is emitted by most fluorescent substances. This is very different from incandescent light, the type of light that is found in traditional light bulbs that is emitted due to high temperatures.

Another phenomenon that is very similar to fluorescence is phosphorescence. In both cases, cold light will be emitted when the light-emitting object is exposed to an external energy source. However, fluorescence differs from phosphorescence in that the fluorescent light ceases immediately after the energy source is removed, while a phosphorescent light will continue glowing for some time after.

This phenomenon occurs when a high energy photon impacts the fluorescent material, or fluorophore, and excites the electrons that make up the atoms of the fluorophore. These electrons are driven to a high-energy state from which they eventually return to their normal, or ground state. During this process, the excess energy can be released as another photon that carries less energy than the one that originally impacted the fluorophore.

This lower energy photon is what our eyes are able to pick up as fluorescent light. The emitted photon may be of a wavelength that is detectable by the naked eye, or it may be of shorter or longer wavelengths and only visible using certain filters. For a given fluorophore, the relationship between the exciting photon wavelength and the emitted photon wavelength is constant. This means that if a given fluorophore is observed under the microscope using a laser of constant power, the color seen through the eyepiece will remain the same.

Fluorescence is used extensively in biochemical and molecular research, as well as in forensic science. For example, DNA can be visualized using a fluorescent compound known as ethidium bromide, which binds to certain types of DNA and allows them to be seen as orange bands under a UV light. Forensic scientists also use the fluorescent nature of certain bodily fluids such as blood, urine, and semen, to find them at a crime scene. These will glow under a UV light, even when they are invisible under natural light. It can also used in painting and other art, which use fluorescent materials and are shown under UV light to create otherworldly effects. Certain collectible items such as gemstones can be identified in this way. For example, some diamonds will glow blue when exposed to a UV light source.

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Discussion Comments
By Oceana — On Nov 27, 2012

I always think of fireflies when I think of fluorescence. I spent many summer nights catching these in my hand, and at the time, I knew them as “lightning bugs.”

They would flash a yellow-green light every few seconds. This is how I could follow them in the dark.

The most magical thing was seeing a swarm of them in one place. I once saw hundreds of them on one tree, and they made it look like a lit Christmas tree!

By giddion — On Nov 26, 2012

Fluorescence has such an intensity. I was always enchanted with glow sticks as a child because of how bright they appeared to be.

The only thing in nature that rivaled their intensity was the sun itself. However, looking at the sun was painful and impossible to do for more than a second or so. I enjoyed being able to stare at glow sticks for a long time without damaging my eyes.

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