Ultraviolet (UV) light is a segment of the electromagnetic spectrum beyond human vision. It lies just past the violet end of the visible light spectrum, with shorter wavelengths and higher energy. Though invisible, UV light is present in sunlight and produced by specialized lamps like black lights. This radiation reveals a hidden world, uncovering details concealed under normal visible light.
The Nature of Ultraviolet Light
Ultraviolet light occupies the electromagnetic spectrum with wavelengths from 10 to 400 nanometers (nm), shorter than visible light but longer than X-rays. Our eyes cannot detect UV radiation directly. The human eye’s lens blocks most UV light from reaching the retina, and photoreceptor cells are sensitive only to the visible spectrum (380 nm to 700 nm). Our visual system lacks the specific photoreceptors to translate UV into a perceived color.
Many animals, however, can see into the UV spectrum. Unlike humans with trichromatic vision, many birds, insects, and fish have tetrachromatic vision, perceiving UV light as an additional color. This ability comes from UV-sensitive photoreceptors or lenses that do not filter out UV wavelengths. For these creatures, the world appears with an expanded palette, revealing information inaccessible to us.
How Objects Appear in UV
Objects interact with ultraviolet light in distinct ways, affecting their appearance under UV illumination. Absorption causes objects to appear dark or black by absorbing UV light. Conversely, objects reflecting UV light appear bright. These differences in absorption and reflection can reveal otherwise invisible patterns.
Fluorescence is another interaction where substances absorb UV light and re-emit that energy as visible light, creating a glow. This occurs because absorbed UV photons excite molecules, which then release energy as longer-wavelength visible light. The glow’s color depends on the material’s fluorescent properties. This mechanism makes many objects appear vibrant under a “black light,” which emits primarily UVA light.
UV in Nature: A Hidden World
Perceiving ultraviolet light plays a significant role in the natural world, guiding biological interactions. Many flowers display “nectar guides” on their petals, visible only in UV light. These markings, often bulls-eye patterns or lines, direct pollinators like bees to nectar and pollen, increasing pollination efficiency.
Many animal species use UV patterns for communication, mate selection, and foraging. Birds, with tetrachromatic vision, exhibit UV-reflective plumage for attracting mates and recognizing their species. Insects like butterflies and bees rely on UV vision for identifying suitable flowers. Some fish use complex UV facial patterns to distinguish between species and individuals. Certain ripe fruits and berries also develop UV-reflective cues, signaling readiness to animals that perceive these wavelengths.
Unveiling the Unseen: Practical Uses of UV Vision
Humans use ultraviolet light to reveal hidden information in various practical applications. Security features in currency, passports, and driver’s licenses incorporate UV-reactive inks and fibers. These elements are invisible under normal light but glow distinctly when exposed to UV, deterring counterfeiting and aiding authentication.
UV light is also a tool in art and forensics. Art conservators use it to detect otherwise imperceptible repairs, touch-ups, or forgeries on paintings. In forensic investigations, UV illumination can reveal biological evidence, such as bodily fluids, which often fluoresce.
Many common household items also exhibit fluorescence under UV light. Laundry detergents, for example, contain optical brighteners that absorb UV light and re-emit it as blue visible light, making white fabrics appear brighter. Tonic water glows blue due to quinine, a compound that fluoresces under UV radiation. Petroleum jelly and some vitamins also glow under UV.