Ultraviolet (UV) light is a form of electromagnetic radiation, residing just beyond the violet end of the visible light spectrum. While invisible to the human eye, this energy interacts with certain materials to reveal hidden details. This unique property allows UV light to uncover information concealed under ordinary illumination.
The Science Behind the Glow
The ability of UV light to make things visible stems primarily from a phenomenon called fluorescence. When UV light, which carries higher energy than visible light, strikes a fluorescent substance, its photons are absorbed by the electrons within the material. This absorption temporarily boosts the electrons to a higher, more energetic state.
However, this elevated energy state is unstable. To return to their original, more stable orbits, these excited electrons release the absorbed energy. This released energy is emitted as light, but at a longer wavelength and therefore lower energy than the absorbed UV light. This re-emitted light often falls within the visible spectrum, making the substance appear to “glow” to the human eye. The glow occurs almost instantaneously after UV exposure.
Substances Revealed by UV Light
Many substances fluoresce when exposed to UV light. Biological fluids such as semen, saliva, and urine can fluoresce, making them detectable in forensic investigations. Blood, while not directly fluorescent, can be made visible through chemicals that react and then fluoresce under UV light.
Minerals and gemstones also frequently display vibrant fluorescence. Examples include fluorite and calcite, which can glow in a wide range of colors like pink, red, blue, or green. Other minerals such as willemite can exhibit a bright green fluorescence, while adamite often glows yellow-green.
Scorpions are well-known for their striking blue-green luminescence under UV light, emanating from a substance within their exoskeleton. Certain inks and dyes are designed to be invisible under normal light but become brightly visible under UV, including invisible inks for secret messages and security markings.
Fabric whiteners, also known as optical brighteners, absorb UV light and re-emit it as blue light, making white fabrics appear brighter and masking yellowing. UV light can also reveal mold and fungi on surfaces, particularly in art conservation where it helps identify imperfections and fungal presence on paintings.
Practical Applications of UV Light
UV light has widespread application across various fields. In currency and document verification, it routinely detects counterfeit items. Banknotes, passports, and identification cards incorporate security features like fluorescent threads or watermarks that glow distinctly under UV, aiding authentication.
Forensic science relies heavily on UV light to uncover hidden evidence at crime scenes. Investigators use it to detect biological fluids that fluoresce under UV illumination, and to identify trace evidence like fibers or enhance latent fingerprints with fluorescent powders.
Art restoration and authentication benefit from UV light. Conservators use it to identify previous repairs, overpainting, and hidden signatures on artworks, as modern paints often fluoresce differently than older pigments. It also helps assess varnish condition, as aged varnishes may appear green or yellow under UV light.
In pest control, UV flashlights locate rodent urine, which contains phosphors that glow brightly, identifying activity pathways and nesting areas. Similarly, it aids general hygiene and cleaning inspections, revealing unseen contaminants or stains.