Final answer:
Fluorescence is the emission of light by a substance as it transitions from an excited state back to the ground state after absorbing a high-energy photon.
Step-by-step explanation:
Fluorescence occurs when a substance emits a photon of light as it transitions from an excited state back to the ground state. During this process, the electron first absorbs a high-energy photon, often in the ultraviolet (UV) range, boosting it to a higher energy level above its ground state. After some nonradiative loss of energy, typically as heat, the electron returns to the ground state by emitting a photon with less energy (longer wavelength) than the one absorbed, resulting in the characteristic glow of fluorescent materials. This property of substances is utilized in applications like fluorescent lighting, where electrical energy excites electrons, and in the fields of biochemistry and molecular biology, where fluorescence aids in various forms of microscopy and imaging.