Luminescence - Fluorescence And Phosphorescence
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Fluorescence and phosphorescence
Luminescence can be divided into categories by duration (fluorescence or phosphorescence) or by the mechanism that creates the light. By definition, fluorescent things stop emitting light very soon (about 10 ns) after the exciting energy is cut off. Phosphorescence continues for longer than fluorescence. Glow-in-the-dark stickers and watch hands that glow are examples of phosphorescence. A less obvious but more exact definition of the difference is that the amount of time phosphorescence continues after the material has been excited may change with temperature, but in fluorescence, this decay time does not change. Also, phosphorescence tends to occur at longer wavelengths than fluorescence.
Fluorescent dyes are included in many clothing detergents to make the clothes appear brighter. Because most organic materials fluoresce when excited by ultraviolet light, fluorescent spectroscopy is used to study organic molecules and atoms by the "fingerprint" of their light emissions: the wavelengths, lifetime, polarization, and brightness of their fluorescence.
A common uses of phosphors (phosphorescent materials) is in televisions and computer monitors: small dots of red, green, and blue phosphors are grouped together on the inner surface of a cathode-ray tube. When electrons generated in the back of the tube hit the phosphors, they absorb the energy and then emit light.