Light emission is the process by which a substance absorbs energy and then releases it as light. This phenomenon is responsible for nearly every source of light we encounter, from the glow of a firefly to the illumination of a lamp. It is a process that occurs across the universe, from the smallest glowing organisms to the most massive, distant stars.
The Fundamental Process of Emission
At the heart of all light emission is a process that occurs on an atomic scale. The atoms that make up all matter have electrons orbiting a central nucleus, and these electrons occupy specific “energy levels,” much like planets in defined orbits around a sun. In their natural, resting state, electrons stay in the lowest, most stable energy level available, known as the ground state.
To create light, external energy must be introduced to the atom. This energy—perhaps from heat or an electrical current—can be absorbed by an electron. This absorption forces the electron to jump from its stable ground state to a higher, less stable energy level, a process called excitation. However, this excited state is temporary, lasting for only a fraction of a second.
Because systems in nature prefer lower energy states, the excited electron will quickly fall back toward its original, stable energy level. As it drops, the atom releases the extra energy it absorbed in the form of a particle of light called a photon. The energy of this photon is exactly equal to the energy difference between the higher, excited state and the lower, stable state, a principle that dictates the specific color of the emitted light.
Incandescence: Light from Heat
One of the most common forms of light emission is incandescence, which is simply light produced from high temperatures. When an object is heated, its atoms begin to vibrate rapidly. This intense thermal agitation provides the energy needed to excite the electrons within the atoms, creating a visible glow.
The color of incandescent light is directly linked to the object’s temperature. A metalworker, for instance, can judge the temperature of a piece of steel by its color, watching it change from a dull red to a brighter orange-white as it gets hotter. This principle is also at work in a traditional incandescent light bulb, where an electric current heats a thin tungsten filament to temperatures between 2,000 and 3,300 Kelvin. At these temperatures, the filament emits a broad, continuous spectrum of light, although most of the energy is released as invisible infrared heat.
Luminescence: Light Without Heat
In contrast to incandescence, luminescence is the emission of “cold light”—light produced by methods other than high temperatures. This process occurs when energy from sources other than thermal vibration excites electrons. Several distinct types of luminescence exist, each triggered by a different form of energy.
Electroluminescence is the result of an electrical current passing through a material. This is the principle behind light-emitting diodes (LEDs). In an LED, electricity causes electrons in a semiconductor material to recombine with “holes” (areas lacking electrons), releasing their excess energy directly as photons.
Other forms of luminescence are triggered by chemical or biological actions. Chemiluminescence is light that originates from a chemical reaction, with the common glow stick being a prime example. Bioluminescence is a similar process that occurs within living organisms, such as fireflies, which use a chemical reaction involving luciferin and the enzyme luciferase to produce their characteristic flashes. Photoluminescence happens when a material absorbs light energy and then re-emits it later, as seen in glow-in-the-dark stars which use phosphors like strontium aluminate to absorb light and then slowly release it over several hours.
Analyzing Light with Emission Spectra
Scientists can learn much about a substance by analyzing the light it emits. When light from an emission source is passed through a prism or a spectroscope, it separates into its component colors. Unlike the continuous rainbow of colors produced by an incandescent source, the light from an excited gas of a single element creates a series of distinct, bright lines against a dark background. This pattern is known as an emission spectrum.
This emission spectrum is unique for every chemical element. Because each element has a distinct arrangement of electrons and energy levels, the transitions its electrons can make are specific, resulting in the emission of photons at exact, predictable wavelengths. The pattern of colored lines serves as a unique “fingerprint” or “barcode” that allows for the identification of that element.
This analytical technique has applications in various scientific fields. Astronomers, for example, analyze the light from distant stars to determine their chemical composition. By matching the star’s spectrum to the known emission spectra of elements, they can identify which elements are present in its atmosphere. Similarly, chemists can use emission spectroscopy to determine the elemental makeup of an unknown sample with high precision.