Hundreds of everyday materials glow under ultraviolet light, from the cash in your wallet to the tonic water in your fridge. The glow happens through a process called fluorescence: a material absorbs UV light (which is invisible to your eyes) and re-emits it as visible light in a fraction of a second. The result is that eerie, vibrant glow you see in a darkened room. Here’s a practical guide to the most common things that fluoresce and why.
Why Things Glow Under UV Light
When UV photons hit certain molecules, they bump electrons into a higher energy state. Those electrons drop back down almost instantly, within about one-billionth to one-millionth of a second, releasing the extra energy as visible light. That’s fluorescence. A related phenomenon, phosphorescence, works the same way but the glow lingers for seconds or even minutes after the UV source is removed because the electrons take a slower path back to their resting state. Glow-in-the-dark stickers are phosphorescent. The bright, instant glow of a blacklight poster is fluorescent.
The color of the glow depends entirely on the molecule doing the absorbing. Different chemical structures release photons at different wavelengths, which is why some things glow blue, others green, and others bright orange under the same UV lamp.
Household Items
Tonic water is the classic example. Quinine, the bitter compound that gives tonic water its flavor, absorbs UV light at around 350 nanometers and emits a vivid blue glow at 450 nanometers. Even a small splash in a glass lights up dramatically under a blacklight.
White paper, clothing, and laundry detergent often glow a bright blue-white. Manufacturers add optical brighteners to these products specifically to make whites look “whiter than white” in daylight. Those same brightening agents fluoresce intensely under UV. White T-shirts at a blacklight party glow for exactly this reason.
Petroleum jelly (Vaseline) glows blue. Many plastics, particularly those containing optical brighteners, glow as well. Some antifreeze formulations glow a bright green because manufacturers add fluorescent dye so leaks are easier to spot. Certain postage stamps, highlighter inks, and neon paints are designed to be fluorescent and will glow vividly.
Food and Drinks
Beyond tonic water, several foods contain naturally fluorescent molecules. Honey glows a golden yellow under UV due to its mix of organic compounds. Olive oil fluoresces because of chlorophyll and other pigments present from the pressing process. Ripe bananas develop fluorescent spots as their skin breaks down, with blue-glowing compounds forming around the brown speckles.
Vitamin B2, also called riboflavin, is one of the strongest natural fluorophores in food. It’s a yellow-colored compound that emits a bright yellow-green glow. Foods rich in riboflavin, like milk, eggs, and nutritional yeast, can show a faint fluorescence when dissolved or spread thin under UV light. If you’ve ever taken a B-complex supplement and noticed your urine turning bright yellow, that excess riboflavin will also make the urine glow under a blacklight.
Bodily Fluids
Several body fluids fluoresce under UV, which is why forensic investigators rely on UV lamps at crime scenes. Semen glows a blue-white, primarily because of proteins containing the amino acid tryptophan and fluorescent oxidation products. Urine glows yellow-green thanks to a cocktail of fluorescent molecules including urobilin, riboflavin metabolites, and porphyrins. Saliva fluoresces faintly, again driven mostly by tryptophan in the enzyme that breaks down starch.
Blood does not glow under UV light on its own, which is a common misconception. Forensic teams use a separate chemical spray (luminol) to detect blood, which works through a different chemical reaction entirely. Sweat residues and fingerprints do fluoresce, however, because of tryptophan and its breakdown products deposited on surfaces by your skin.
Minerals and Rocks
Fluorescent minerals are some of the most spectacular things you can view under UV. The word “fluorescence” itself comes from the mineral fluorite, which typically glows blue under longwave UV. Calcite is another standout, capable of producing a rainbow of colors depending on its trace impurities. Willemite glows an intense green under shortwave UV and is one of the brightest fluorescent minerals known.
Other notable examples:
- Sodalite: glows vivid orange under longwave UV
- Scheelite: bluish-white under shortwave UV, historically used by tungsten prospectors to locate ore deposits
- Corundum (ruby and sapphire): glows red under longwave UV
- Opal: can glow chartreuse or yellow
- Aragonite: yellow or white, sometimes with a lingering greenish phosphorescent afterglow
The type of UV matters for minerals. Shortwave UV (around 254 nm) and longwave UV (around 365 nm) excite different minerals, and some specimens only respond to one or the other. Serious mineral collectors often carry both types of lamp.
Animals and Insects
Scorpions are the most famous fluorescent animal. Their exoskeletons contain compounds in the outer cuticle that glow a bright blue-green under UV, a trait that persists even in fossils millions of years old. Scientists still debate the purpose, though it may help scorpions detect UV light and avoid overexposure.
Platypuses fluoresce a blue-green under UV, discovered in museum specimens in 2020. Flying squirrels glow pink. Many amphibians are fluorescent as well: a 2020 study in Scientific Reports documented biofluorescence across salamanders and other amphibians, suggesting the trait is far more widespread in land animals than previously thought. Most earlier research had focused on marine life, where biofluorescence is common in corals, jellyfish, and ray-finned fishes.
Some fungi glow too. Certain species of mushroom and lichen contain fluorescent pigments, and many spiders fluoresce green or blue at their joints.
Plants and Chlorophyll
Fresh green leaves and algae glow a deep red under UV light. This is chlorophyll fluorescence. Chlorophyll absorbs light across the visible spectrum for photosynthesis, but a small percentage of that absorbed energy gets re-emitted as red to far-red light (approximately 680 to 740 nm). Under a UV lamp in a dark room, this gives leaves, grass clippings, and even green smoothies a striking crimson glow.
Plant scientists actually use chlorophyll fluorescence as a diagnostic tool. Because stressed plants handle light energy differently, measuring the intensity and pattern of the red glow can reveal heat stress, drought stress, UV damage, salt stress, and even pollution exposure before visible symptoms appear.
Money and Security Documents
Modern currency uses fluorescent security features to deter counterfeiting. U.S. bills ($5 and above) have a clear security thread embedded in the paper that glows a specific color under UV. Each denomination is different: $5 bills glow orange, $10 bills glow blue, $20 bills glow light red or pink, $50 bills glow yellow, and $100 bills glow green. The $1 and $2 bills do not have this thread.
The paper itself is another clue. Genuine U.S. currency is made from 75% cotton and 25% linen with tiny red and blue fibers scattered throughout. Standard copy paper contains optical brighteners that glow bright blue-white under UV, while authentic currency paper does not. A counterfeit bill printed on regular paper will light up like a sheet of notebook paper under a blacklight, making it immediately obvious.
Passports, driver’s licenses, credit cards, and event tickets also incorporate fluorescent inks and patterns visible only under UV. These hidden designs are difficult and expensive to reproduce, making UV lamps a quick first check for document authenticity.
Medical Uses
Dermatologists use a handheld UV device called a Wood’s lamp to diagnose skin conditions. Different infections produce different fluorescent colors on the skin’s surface. Certain fungal infections glow blue-green. A common bacterial skin infection called erythrasma produces a distinctive coral pink fluorescence. Some yeast infections show up as yellow or orange. The lamp can also reveal pigmentation disorders and help map the borders of certain skin lesions that aren’t visible in normal light.
UV Safety Tips
Most consumer blacklights emit longwave UV (around 365 to 405 nm), which is the least energetic type of ultraviolet radiation. Brief, casual exposure from a blacklight is not a significant skin risk, but your eyes are more vulnerable. Protective UV-filtering glasses are recommended whenever you’re using a UV lamp for extended viewing, particularly at closer distances. Even “near-UV” lights marketed at 400 to 405 nm can emit some shorter UV wavelengths, so eyewear is a sensible precaution regardless of the label. Shortwave UV lamps (254 nm), the kind used for mineral collecting, carry a higher risk and should always be used with proper eye protection and minimal skin exposure.