Ultraviolet light is outside of the visible light spectrum. Fluorescence, or fluorescent materials, refers to chemicals that illuminate under ultraviolet black light. Fluorescence appears naturally in some compounds, such as rocks or minerals, or you can add it intentionally. Fluorescent chemicals absorb ultraviolet black light, causing them to glow or become illuminated.
Some body fluids naturally glow under ultraviolet light because they contain minerals with a small amount of phosphorus or other fluorescent chemicals. Ultraviolet black light may illuminate blood, urine, semen, saliva or even sweat. Forensic scientists use ultraviolet black light at crime scenes to reveal any body fluids from criminals for DNA samples. If you place any body under ultraviolet black light, it may appear to glow slightly due to sweat and fluids under the skin.
Some laundry detergents may glow under ultraviolet black light and some clothes may illuminate if washed in the detergent. Detergents used to clean white clothes are more likely to glow white, as are white clothes themselves. The detergents contain fluorescent chemicals or dyes to make white clothes appear brighter under normal light. When placed under ultraviolet black light, the detergent will have a blue illumination. Any residue left from the detergent on clothes will also make them glow blue.
Teeth Whitening Products
Teeth-whitening toothpaste or teeth-whitening chemicals may also contain fluorescent dye or colouring to make teeth appear whiter. Much like detergent, teeth-whitening products dye teeth to make them appear less yellow in normal light. But teeth whitened by products with fluorescent compounds will illuminate under ultraviolet black light. Some tooth-whitening chemicals may not have the same effect. For example, some teeth-whitening bleaches remove yellow stains completely and do not cover the stains with fluorescent chemicals to make them appear less yellow.
Postage Stamps or Bills
Some postage stamps and paper money purposely contain fluorescent chemicals to prove their authenticity. For example, some £13 bills contain a security strip on the left side that glows under ultraviolet black light for authentication. Postage stamps have no specific colour to prove authenticity, but some, such as first class stamps in the United Kingdom, have fluorescent colouring around the edges.