Short answers, pulled from the story.
Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. A photon is absorbed by a molecule, exciting it to a higher energy level, and light is emitted as the molecule returns to a lower energy state. The emitted light usually has a longer wavelength and lower energy than the absorbed radiation, a gap known as the Stokes shift.
Fluorescent materials generally stop glowing almost immediately when the radiation source stops, while phosphorescent materials continue to emit light for some time afterward. The difference results from quantum spin effects. Fluorescence occurs when the excited and final states have the same spin multiplicity, while phosphorescence involves a change to a triplet state and slower decay.
George Gabriel Stokes coined the word fluorescence in his 1852 paper on the "Refrangibility" of light. He took the name from fluor-spar, the mineral fluorite, comparing it to how opalescence is derived from opal. He used it to describe how fluorspar, uranium glass, and other substances change invisible light beyond the violet end of the spectrum into visible light.
Fluorescence appears across all kingdoms of life and has been studied most extensively in cnidarians and fish, including eels, gobies, cardinalfishes, and triggerfishes. The polka-dot tree frog was the first fluorescent amphibian discovered, in 2017. Other fluorescent animals include scorpions, spiders, swallowtail butterflies, budgerigars, platypuses, and all three species of North American flying squirrels.
A fluorescent lamp uses a glass tube holding a partial vacuum and a small amount of mercury. An electric discharge makes the mercury atoms emit mostly ultraviolet light, and a phosphor coating lining the tube absorbs that ultraviolet light and re-emits visible light. Fluorescent lights were first available to the public at the 1939 New York World's Fair and are more energy-efficient than incandescent lighting.
Fluorescence is used in mineralogy, gemology, medicine, chemical sensors, fluorescent labelling, dyes, biological detectors, cosmic-ray detection, vacuum fluorescent displays, and cathode-ray tubes. It powers fluorescent and LED lamps, lasers, DNA detection with dyes such as ethidium bromide, fluorescent road signs, and optical brighteners that whiten fabric and paper. A fluorometer can measure fluorescent molecule concentrations as low as one part per trillion.