Fluorescence is a phenomenon where cold light is emitted by a substance when exposed to an energy source. It differs from phosphorescence as the light stops immediately after the energy source is removed. Fluorescence is used in research, forensics, and art, and can be used to identify certain materials.
Fluorescence is an optical phenomenon that manifests itself as incandescent light. We see this every day in energy-saving compact fluorescent bulbs that are used in many homes and in some types of glow sticks, but also in nightclubs. It is often referred to as “cold light” because most fluorescent substances emit very little heat. This is very different from incandescent light, the type of light found in traditional light bulbs that is emitted due to high temperatures.
Another phenomenon 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 fluorescent light ceases immediately after the energy source is removed, whereas a phosphorescent light will continue to glow for some time afterward.
This phenomenon occurs when a high-energy photon strikes the fluorescent material, or fluorophore, and excites the electrons that make up the atoms of the fluorophore. These electrons are brought to a high-energy state from which they eventually return to their normal or ground state. During this process, excess energy can be released as another photon carrying less energy than the one that originally hit the fluorophore.
This low-energy photon is what our eyes are able to perceive as fluorescent light. The photon emitted can be of a wavelength detectable with the naked eye, or it can be of shorter or longer wavelengths and visible only using certain filters. For a given fluorophore, the relationship between the wavelength of the exciting photon and the wavelength of the emitted photon is constant. This means that if a given fluorophore is viewed under a 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 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 crime scenes. These will glow under a UV light, even when invisible in natural light. It can also be used in painting and other artworks, which use fluorescent materials and are displayed under UV light to create otherworldly effects. Some collectibles such as gemstones can be identified this way. For example, some diamonds will glow blue when exposed to a UV light source.
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