Luminescence describes the emission of light from a substance for any reason other than heat. This phenomenon is often called “cold light” because it can occur at normal or cool temperatures. It stands in direct contrast to incandescence, which is light generated from high temperatures, like a hot stove burner or the filament in a traditional light bulb. Incandescent light sources are inefficient, losing a significant amount of energy as heat.
The Science of Luminescence
Luminescence is a process that begins when an external energy source interacts with the atoms of a substance. This energy causes an electron within an atom to jump from its stable “ground state” to a higher “excited state”. This excited state is unstable, and the electron cannot remain there for long.
To return to its stable ground state, the electron must release the extra energy it absorbed. It accomplishes this by emitting a particle of light, known as a photon. The energy of this photon determines the color of the light we see. This entire process allows for the creation of light without the high temperatures associated with incandescence.
An analogy is throwing a ball up a flight of stairs. Lifting the ball gives it potential energy, moving it to a higher, less stable position. When the ball tumbles back down the stairs, it releases that stored energy, partly as sound. In luminescence, an electron takes the place of the ball, and the energy is released as light instead of sound.
Types of Luminescence
The type of luminescence is defined by its energy source. Different sources lead to distinct forms of this phenomenon.
Photoluminescence occurs when the energy is supplied by light itself. This category is further divided into two types: fluorescence and phosphorescence. Fluorescence happens when a material absorbs light and emits it almost instantaneously, ceasing to glow as soon as the light source is removed. This immediate effect is what causes a highlighter to appear so bright under a blacklight.
Phosphorescence involves a much slower release of light. After the external light source is turned off, a phosphorescent material will continue to glow, sometimes for minutes or even hours. This delayed emission is due to a “forbidden” energy state transition that traps the energy for a longer period. Glow-in-the-dark stars and toys are common examples of phosphorescence.
Chemiluminescence is light produced from a chemical reaction. The classic example is a glow stick, where bending the plastic tube breaks an inner vial, mixing two chemical solutions. This mixture initiates a reaction that releases energy in the form of light rather than heat. A specific form of this is bioluminescence, which is chemiluminescence that occurs within a living organism.
Finally, electroluminescence is generated by an electric current or a strong electric field. When electricity passes through certain semiconductor materials, it excites the electrons, causing them to release photons. This direct conversion of electrical energy to light is the principle behind many modern technologies.
Luminescence in Nature
Nature provides captivating displays of luminescence, primarily through bioluminescence. Organisms use this light for communication, defense, or predation where sunlight is scarce.
Fireflies are a familiar example, using timed flashes of light to attract mates. The pattern and color of the flashes are species-specific, acting as a calling card in the dark. This light-based language results from a chemical reaction in their abdomens.
In the deep ocean, where sunlight cannot penetrate, bioluminescence is a common survival tool. The anglerfish famously uses a dangling, luminous lure to attract unsuspecting prey. Other creatures, like certain species of dinoflagellates, create a startling burst of light when disturbed, a defensive mechanism believed to deter predators by creating a glowing trail that can attract even larger threats to the initial attacker.
Luminescence in Technology
The principles of luminescence are used to create technologies for lighting, displays, and safety. Electroluminescence is the foundation of modern solid-state lighting.
Light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) use an electric current to stimulate a semiconductor material, causing it to emit light. This technology is now ubiquitous in everything from household light bulbs and traffic signals to the vibrant, high-contrast screens of smartphones and televisions.
The persistent glow of phosphorescent materials is used for safety and novelty. Glow-in-the-dark paints and plastics are applied to emergency exit signs, watch dials, and children’s toys. This application of phosphorescence ensures visibility in low-light conditions without the need for a constant power source.
Chemiluminescence is used in specialized fields like forensic science. Investigators use a chemical called luminol, which reacts with the iron in hemoglobin. When sprayed at a crime scene, luminol will glow a faint blue where it contacts even microscopic traces of blood, revealing evidence that would otherwise be invisible. This reaction helps uncover hidden details.
Fluorescence also has numerous technological uses. It is the principle behind fluorescent lamps, where an electric current excites mercury vapor to produce ultraviolet (UV) light. This invisible UV light then strikes a fluorescent coating on the inside of the tube. This same principle is used in security features on banknotes and passports, where fluorescent fibers or inks become visible only under a UV light source.