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What common substances glow when placed under a black light?


Jason 3/02/99

Vocabulary
fluorescence*
ultraviolet radiation*
visible light*
wavelength*
Fluorescent substances absorb short-wavelength light (such as the ultraviolet radiation given off by a black light) and re-emit it almost instantaneously (usually in 10-6 to 10-9 seconds). The emitted light often has a longer wavelength than the radiation that was originally absorbed.

Not all substances can fluoresce. Fluorescent substances usually have highly rigid molecular structures with electrons that are delocalized over the entire framework of the molecule. Examples of common materials that contain fluorescent molecules are:

  • White paper is treated with fluorescent compounds to make it appear brighter. Historical documents that have been forged can sometimes be detected by placing them under a black light to see if they fluoresce; fluorescers have only been used in paper made after about 1950 [1].
  • Club soda or tonic water contains quinine, a bitter-tasting fluorescent compound that glows blue-white under a black light.
  • Vitamins (specifically vitamin A, thiamine, niacin, and riboflavin) fluoresce strongly. A small piece of a vitamin B-12 tablet crushed and dissolved in vinegar glows a bright yellow under UV light.
  • Chlorophyll fluoresces with a blood red color. Grind spinach or swiss chard with ethanol and filter the resulting chlorophyll extract. Bright white light can be used to drive the fluorescence. Place the extract in a petri dish on top of an overhead projector lamp in a darkened room to get the full effect.
  • Many biological fluids contain fluorescent molecules. Forensic scientists use UV lights on crime scenes to detect fluorescence from blood, urine, or semen. Some marine organisms (notably jellyfish) produce proteins that are very intensely fluorescent.
  • Antifreeze fluid contains additives that fluoresce. Investigators sometimes use black lights to look for antifreeze splashes on pavement and vehicles to reconstruct automobile accident scenes [2].
  • Laundry detergents contain fluorescers to make the whites "whiter than white". Detergent residues on white clothing causes it to glow blue-white under a black light. These fluorescers sometimes make white clothing appear blue in color photographs.
  • Dental enamel and whiteners also contain fluorescent compounds that fill in the blue part of the spectrum to prevent the enamel from appearing too yellow [3].
  • Postage stamps are printed with inks that contain fluorescent dyes.
  • Some common minerals and gemstones fluoresce, including gypsum, talc, opal, agate, quartz, and amber. For a complete list see Amethyst Galleries' Fluorescent, Phosphorescent, Thermoluminescent and Triboluminescent Minerals page.

Notes

  1. Authentication of Manuscripts and Historical Documents, S. D. Miller, Duke University.
  2. Unified News, Spring 1996. Includes some interesting applications of fluorescence.
  3. The Science and Art of Color (website, now offline) (in dentistry). Jack D. Preston, USC School of Dentistry.

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! What common substances glow when placed under a black light?

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Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/consumer/faq/what-are-fluorescent-compounds.shtml