Atoms and molecules can emit light through a process called emission or luminescence. There are different mechanisms through which this occurs, the most common being:
Incandescence: Atoms or molecules can emit light when heated to high temperatures. As they gain thermal energy, the electrons within them become excited and jump to higher energy levels. When these excited electrons return to their original energy levels, they release the excess energy in the form of light. This process is how objects such as a heated metal filament in an incandescent light bulb emit visible light.
Fluorescence: Fluorescence occurs when an atom or molecule absorbs photons (light particles) of a specific energy, promoting its electrons to higher energy levels. The excited electrons quickly return to their lower energy levels, emitting light of lower energy (longer wavelength) than the absorbed photons. This emitted light is often of a different color than the absorbed light. Fluorescent materials, like certain dyes or the phosphors used in fluorescent lamps, absorb ultraviolet (UV) light and emit visible light.
Phosphorescence: Phosphorescence is similar to fluorescence but involves a longer-lived emission of light. After absorbing photons, the excited electrons in the atom or molecule undergo a change in their spin configuration, leading to a longer-lasting excited state. As the electrons return to their original energy levels, they emit light over a longer period, sometimes lasting minutes or even hours. Glow-in-the-dark materials exhibit phosphorescence.
Bioluminescence: Bioluminescence is the emission of light by living organisms, such as fireflies or certain deep-sea creatures. It is a complex process involving enzymatic reactions. In brief, a molecule called luciferin reacts with an enzyme called luciferase, along with oxygen and adenosine triphosphate (ATP), to produce an excited state and subsequent emission of light.
In all of these cases, the emission of light is a result of the energy changes occurring within the atoms or molecules. The specific wavelengths and colors of light emitted depend on the energy differences between the electronic energy levels involved in the transitions.