When exposed to a black light, many substances reveal a hidden glow, leading to curiosity about whether common bodily fluids, such as saliva, also exhibit this characteristic. Saliva does not produce a strong, noticeable glow under a black light. While it may possess some fluorescent properties, this effect is very faint and not as prominent as that seen with other materials. Black lights emit ultraviolet (UV) light, which causes certain compounds to fluoresce.
The Science of Saliva and UV Light
Fluorescence is a phenomenon where a substance absorbs ultraviolet light and then re-emits it as visible light. This process occurs when molecules within a material, known as fluorophores, absorb energy from UV radiation, causing their electrons to become excited. As these excited electrons return to their original energy state, they release the absorbed energy as light at a longer wavelength, making it visible. This occurs due to a slight energy loss during the process.
Human saliva is primarily composed of water, making up about 99.5% of its volume. The remaining components include electrolytes, mucus, various proteins, and enzymes, such as alpha-amylase. While saliva contains organic compounds and enzymes with fluorescent properties, such as alpha-amylase and tryptophan, their concentrations mean the glow is not pronounced. For instance, alpha-amylase in saliva has been observed to emit light in the 345-355 nm range when excited by UV light. This faint fluorescence means saliva does not stand out when illuminated by a black light.
Common Fluorescent Substances Under Black Light
Many everyday items contain compounds that cause them to glow under ultraviolet light. Laundry detergents, for example, often include optical brighteners designed to make clothes appear whiter and brighter by absorbing UV light and re-emitting it as blue light. Tonic water exhibits a distinct blue-white glow because of quinine, a bitter compound added during its production. Quinine molecules absorb UV light and re-emit it as visible light.
Certain vitamins, particularly some B vitamins like riboflavin and B-12, also display fluorescence under black light. Scorpions are known for their blue-green glow under UV light, attributed to a substance within their exoskeleton’s hyaline layer. Scientists investigate its purpose, with theories suggesting it aids UV detection, mate finding, or prey confusion.
Other bodily fluids also react differently to UV light. Urine, for instance, can glow due to the presence of compounds such as urea, creatinine, and riboflavin. Semen fluoresces due to its protein and enzyme content, including prostatic acid phosphatase, flavin, or choline proteins. In contrast, blood does not naturally fluoresce under black light; instead, it appears dark or absorbs the UV light. Forensic scientists often use a chemical called luminol, which reacts with blood components to produce a blue chemiluminescent glow, allowing for its detection.