Fluorescence is the phenomenon where materials absorb energy and instantly re-emit it as visible light. Ultraviolet (UV) light, often called black light, is invisible but carries high energy. When this energetic light strikes certain substances, it causes them to glow. The specific color that appears, such as a rich red, depends entirely on the material’s unique chemical composition and structure. This transformation reveals specific details about the atomic makeup of the object, whether it is a mineral, an organic molecule, or a synthetic dye.
How Ultraviolet Light Causes a Red Glow
The mechanism involves the excitation and relaxation of electrons within a material’s atoms. When a high-energy UV photon strikes a fluorescent substance, it is absorbed, causing an electron to jump from its stable ground state to a higher, unstable excited state. The electron immediately loses some of this acquired energy through non-radiative processes, often as heat or molecular vibrations, before returning to its ground state.
The electron then completes its return to the ground state by emitting the remaining energy as a new photon of light, which is the visible glow. This light is always of a longer wavelength and lower energy than the absorbed light, a principle known as the Stokes shift. Because the initial absorbed UV light is high in energy, the resulting visible light shifts toward the lower-energy end of the visible spectrum.
Red light possesses the longest wavelength and the lowest energy of all visible colors (620 to 750 nanometers). A material that fluoresces red under UV light therefore requires a large energy gap between the high-energy UV it absorbs and the low-energy red light it emits. The material’s specific atomic structure dictates this large energy difference, resulting in a deep red color.
Geological Examples of Red Fluorescence
In minerals, the red glow under UV light often results from trace impurity elements acting as “activators” within the crystal lattice. The ruby, a variety of corundum, is a famous example that exhibits strong red fluorescence. This intense glow is caused by minute amounts of chromium ions (Cr³⁺) substituting for aluminum in the crystal structure. When UV light excites these chromium atoms, they re-emit the energy as deep red light, which enhances the gem’s natural color.
Calcite, a common calcium carbonate mineral, also frequently glows red. The red or pinkish-red fluorescence in calcite is activated by the presence of manganese (Mn²⁺) ions within the structure. The concentration of this manganese impurity is crucial; too little results in no glow, and too much can “quench” the fluorescence. This response is often most pronounced under shortwave UV light.
Less common minerals also display this coloration, sometimes depending on the UV wavelength used. Certain feldspars, particularly those in low-iron igneous rocks, can fluoresce in shades of red or violet-red. Smithsonite, a zinc carbonate, exhibits red fluorescence under both longwave and shortwave UV light. The presence of these activators determines the red emission in these materials.
Biological and Organic Sources
The red glow in the organic world is most famously demonstrated by chlorophyll, the pigment that makes plants appear green. Chlorophyll is an efficient light-harvesting molecule, but when isolated from the living plant or exposed to excessive UV, its energy-transfer system is interrupted. Instead of channeling absorbed light energy into photosynthesis, the molecule releases the excess energy as fluorescence.
When a chlorophyll extract, such as crushed spinach leaves in alcohol, is exposed to UV light, it emits a distinct, deep red light, peaking around 680 and 740 nanometers. This occurs because the electrons excited by the UV light cannot pass their energy down the photosynthetic electron transport chain. The rapid release of this energy as a red photon indicates that the molecule is no longer functioning within a living cell.
Beyond plant life, other organic compounds known as porphyrins can also be a source of red fluorescence. Porphyrins are a group of organic molecules that include the core structure of heme in blood and chlorophyll. In biological contexts, porphyrins naturally accumulate in certain tissues and metabolites. For instance, accumulation in teeth or bones can cause them to show a reddish glow under UV light, highlighting specific biological processes.