Ultraviolet (UV) light allows us to perceive details invisible to the unaided human eye. This form of electromagnetic radiation exists just beyond the violet end of the visible light spectrum. While our eyes cannot directly detect UV light, specialized tools can harness its properties to reveal hidden information.
How UV Light Reveals Hidden Details
The ability of UV light to reveal hidden details primarily stems from a phenomenon called fluorescence. Fluorescence occurs when certain materials absorb high-energy UV light and then almost immediately re-emit it as lower-energy visible light. This re-emitted visible light is what we observe as a glow, making otherwise invisible substances apparent. The color of the glow depends on the chemical composition of the fluorescing material.
Some substances, however, might absorb UV light without re-emitting it as visible light, appearing dark under UV illumination. This absorption also provides valuable information by indicating the presence of materials that do not fluoresce. This process is distinct from phosphorescence, where light emission continues for a period after the UV source is removed.
Everyday Uses of UV Light
A widespread application is in verifying the authenticity of currency. Many banknotes incorporate security features, such as specific inks or security threads, that glow uniquely under UV light, making it difficult for counterfeiters to replicate. For instance, a genuine US $20 bill shows a bright green “20” under UV light.
UV light is also used in detecting pet stains on carpets and upholstery. Many biological fluids contain substances that fluoresce when exposed to UV light, revealing dried stains that are otherwise imperceptible. Similarly, UV lights are employed in hotel hygiene inspections to identify unseen bodily fluids or residues on surfaces like bedding and bathrooms, indicating areas that require further cleaning.
Shining a UV light on scorpions causes them to glow a vivid blue-green. This property is attributed to compounds in their exoskeleton, making them easier to spot in dark environments.
Specialized Applications of UV Light
- In forensic investigations, UV light is a tool for detecting bodily fluids, which often fluoresce. It can also reveal latent fingerprints, fibers, and other trace evidence at crime scenes that are invisible to the naked eye. Forensic scientists use different UV wavelengths to enhance the visibility of various types of evidence.
- Art authentication and restoration benefit from UV light by revealing repairs, overpaints, and forgeries. Different pigments and restoration materials react distinctly under UV, allowing experts to differentiate original artwork from later alterations.
- In medicine, a Wood’s lamp, which emits long-wave UV light, assists in diagnosing various skin conditions. It can identify certain bacterial or fungal infections, and pigment disorders, as these conditions cause the skin to fluoresce or change color in characteristic ways.
- Industrial settings also utilize UV light for leak detection. Fluorescent dyes added to fluids in closed systems glow brightly under UV light, pinpointing leaks that would otherwise go unnoticed.