Fluorescence

Fluorescence is light emission from a substance after it absorbs electromagnetic radiation. It usually stops almost immediately when the excitation source is removed.

Jablonski diagram: An energy-level diagram showing electronic states and radiative and nonradiative transitions in a molecule. It maps the excited-state pathways that produce fluorescence.
Fluorescence microscopy: A microscopy technique that uses fluorescent labels or intrinsic fluorescence to form images. It turns emitted light into spatially resolved images of cells and materials.
Phosphorescence: Light emission that persists after excitation because excited states decay through spin-forbidden transitions. Its afterglow contrasts with fluorescence's near-immediate decay when excitation ends.
Photon: A quantum of electromagnetic radiation that carries energy and momentum. Absorption and emission occur through interactions involving individual photons.
Fluorescein: A synthetic fluorescent dye that emits green light and is widely used as a tracer and biological stain. Its bright green emission makes it a classic example of a small-molecule fluorophore.
Stokes shift: The difference between the wavelengths of absorbed light and emitted light, usually with emission at a longer wavelength. Energy lost before emission explains why fluorescence often appears red-shifted from its excitation.
Fluorescence spectroscopy: A technique that measures light emitted by a sample after excitation at selected wavelengths. It uses emission spectra and intensity to identify or quantify fluorescent substances.
Chemiluminescence: Light produced by a chemical reaction without requiring prior absorption of light as the excitation source. It creates excited states chemically rather than by optical absorption.
Electromagnetic spectrum: The complete range of electromagnetic radiation, ordered by frequency or wavelength. Excitation and fluorescent emission occupy particular regions of this spectrum.
Green fluorescent protein: A protein originally isolated from the jellyfish Aequorea victoria that fluoresces green when excited by blue or ultraviolet light. It became a widely used genetically encoded marker in biological research.

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