Black lights possess a unique ability to reveal what is otherwise invisible to the human eye. These specialized lights emit ultraviolet (UV) radiation, which interacts with certain substances. This interaction can cause hidden patterns or materials to become brightly visible.
The Truth About Sweat and Black Light
Human sweat itself does not significantly glow under a black light. While sweat contains water and various dissolved solids like salts and urea, these components do not fluoresce on their own. Therefore, if you observe a glow associated with sweat, it is likely not due to the sweat itself.
Any perceived glow from sweat is commonly attributed to other substances present on the skin or within the sweat. Bacteria, common on human skin, can metabolize sweat components and produce fluorescent byproducts. Additionally, residues from laundry detergents on clothing, which contain optical brighteners, can glow when exposed to UV light, leading to the mistaken impression that sweat is fluorescing. Other bodily fluids mixed with sweat can also contribute to a glow.
Why Some Substances Appear to Glow
The phenomenon that causes certain materials to glow under black light is called fluorescence. A black light emits ultraviolet (UV) light, which is invisible to humans.
When UV light strikes a fluorescent substance, its atoms absorb the UV energy. This absorbed energy excites electrons to a higher energy level. As these excited electrons return to their original, lower energy state, they release the absorbed energy as visible light. Because some energy is lost during this process, the emitted light has a longer wavelength and lower energy than the absorbed UV light, making it visible as a glow. This process occurs almost instantaneously, meaning the glow stops as soon as the black light is removed.
Other Common Substances That Glow
Many common substances exhibit fluorescence under a black light. Tonic water, for example, glows a distinct blue because it contains quinine, a compound that strongly fluoresces when exposed to UV light. Certain vitamins, such as Vitamin A and B vitamins, are also known to be highly fluorescent. A crushed Vitamin B-12 tablet dissolved in vinegar can glow bright yellow under a black light.
Bodily fluids such as urine, semen, and saliva can also fluoresce. Urine glows due to the presence of phosphorus, broken-down proteins, and metabolites. Proteins and other organic compounds within semen and saliva are responsible for their fluorescent properties.
Laundry detergents often contain optical brighteners that absorb UV light and re-emit it as blue light, making white clothes appear brighter and causing them to glow under black light. Petroleum jelly, due to its hydrocarbon content, can also exhibit a bright blue glow.