Luminescence is a process by which a substance emits light without becoming noticeably heated. The term comes from the Latin word “lumen”, meaning “light.” In contrast, incandescence is light resulting from heating a material so that it emits blackbody radiation.
Early humans knew about light from some fungi and the aurora borealis. The phenomenon was formally observed in the early 1600s, with the discovery of the “Bologna Stone”. This barium sulfide-based substance, discovered by an Italian shoemaker and alchemist named Vincenzo Cascariolo, would glow in the dark after exposure to sunlight.
In the 19th century, British scientist Sir George Gabriel Stokes made significant strides in understanding this phenomenon. He coined the term “fluorescence” to describe the glow of fluorspar and uranium glass under ultraviolet light. The broader term “luminescence” was introduced in 1888 by German physicist Eilhard Wiedemann.
How Luminescence Works
At a molecular level, luminescence occurs due to electron transitions. A material absorbs energy, exciting its electrons into higher energy levels. As these electrons return to their normal state, they emit energy in the form of light. The color of the light depends on the energy difference between the excited and normal state, which is unique to each substance.
Categories and Types of Luminescence
There are different categories of luminescence. They depend on the cause of the excitation, the duration of the emission, and the nature of the excited state. Here are the main categories:
- Photoluminescence: This is light emission caused by the absorption of photons. The absorbed energy excites electrons, which emit lower-energy photons when they return to a more stable state. There are two main types of photoluminescence:
- Fluorescence: Fluorescence occurs when a substance absorbs photons and re-emits them very quickly, within nanoseconds. An everyday example is a fluorescent highlighter pen, which glows under UV light.
- Phosphorescence: Phosphorescence is similar to fluorescence, but the substance re-emits the absorbed photons over a longer period, resulting in a sustained glow even after the removal of the exciting source. Glow-in-the-dark stars are a common example.
- Chemiluminescence: This is light produced as a result of a chemical reaction. An everyday example is the light from a glow stick, where a chemical reaction makes the stick glow.
- Bioluminescence: This is a form of chemiluminescence found in certain living organisms, allowing them to produce and emit light. Fireflies, some fungi, and many deep-sea creatures are bioluminescent. Technically, bioluminescence is a form of chemiluminescence that occurs in living organisms.
- Electroluminescence: This is light produced in response to the passage of an electric current or strong electric field through a material. The screens of OLED TVs, night lights, and some types of digital displays use this principle. The aurora borealis is a natural example of electroluminescence.
- Thermoluminescence: This is light produced upon heating of a material. It’s used in archaeology for dating ancient artifacts. This differs from incandescent light produced by heat.
- Cryoluminescence: In contrast to thermoluminescence, cryoluminescence is the emission of light when a material is cooled. Wulfenite is an example of a mineral that displays this type of luminescence.
- Triboluminescence: Triboluminescence is light produced as a result of friction or crushing of a material. It’s often seen when crushing sugar or certain types of crystals.
- Radioluminescence: This is light from the bombardment with ionizing radiation. An example is the glow of radium dials on old watches and clocks. Tritium lights work in much the same way, where radiation makes a phosphor glow.
Uses and Applications of Luminescence
Luminescence is useful living organisms and in various technologies.
- In nature, marine organisms use bioluminescence for hunting, communication, and self-defense. Fireflies use it to attract mates, and glowworms to trap prey.
- Researchers use luminescence for tracing biological processes and dating the age of certain materials.
- Commercial uses include all kinds of lighting solutions.
- Artists and entertainers often use luminescence.
Advancements in Luminescence
Recent advances in nanotechnology and materials science produce new luminescent materials with unique properties. For instance, quantum dots are tiny particles that emit light of different colors based on their size. They find use in cutting-edge display technology to enhance color accuracy and brightness. As another example, genetically engineered luminescent plants are an option for sustainable, energy-efficient lighting. In medicine, luminescent markers hold promise for highlighting cancerous cells. In the field of safety and security, luminescent materials offer better visibility in low light conditions.
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