Phosphorescence is a type of light that absorbs energy from surrounding light waves and emits it in the dark. It differs from bioluminescence and fluorescent light and is measured through phosphorescence spectroscopy. Early glow-in-the-dark materials contained radium, but today they are made from less toxic substances.
Phosphorescence is a type of light, or luminescence, without heat. A phosphorescent object absorbs light for a period of time and then radiates that light for another period of time. Common objects, such as alarm clock hands, glow sticks, road signs, and decals have glow-in-the-dark paint or a glow-in-the-dark solution. The phosphorescent material in these objects absorbs energy from surrounding light waves and emits the light radiation in the dark.
Classified as a type of photoluminescence, the tiny molecules in phosphorescent materials absorb the energy of light photons. Once the molecules have absorbed the photons, they radiate that light. Radiant phosphorescent light is dimmer than absorbed light and usually only lasts a short time. This duration and strength of the light output is called the decay process and is a factor in measuring the length and strength of the radiant light in phosphorescent materials. Unlike other lights, such as sunlight and incandescent light, phosphorescent light does not produce heat.
Phosphorescence is sometimes confused with bioluminescence and fluorescent light. Bioluminescence is the light produced by living marine creatures, such as copepods, anglerfish or jellyfish. This type of glow is produced by a chemical reaction within the creature’s body, not by the absorption of light photons. Phosphorescent light differs from fluorescent light in that fluorescent light does not continue to emit light after absorbing photons.
Scientists have discovered how to measure the absorption and emission rates of phosphorescent light by measuring the decay rate of light emissions. This study of phosphorescent radiation is called phosphorescence spectroscopy. French scientist Henri Becquerel discovered radioactivity while studying phosphorescence in 1896. Two years later, Marie Curie and her husband, Pierre, isolated radium from uraninite, a phosphorescent material that glows blue in the absence of light.
The first dials of wristwatches and alarm clocks were painted with radium, containing low doses of radioactivity. The dial painters who made these products, however, were exposed to greater quantities and suffered from radioactive disease and cancer. In 2011, glow-in-the-dark materials are created from the less toxic tritium, zinc sulfide, and strontium aluminate. Glow-in-the-dark pigments usually appear pale green or beige in normal light, but emit green, red, and blue pigments in the dark.
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