# Emission spectrum An emission spectrum is the set of wavelengths a substance gives off when its electrons fall from higher to lower [[Electron_shell|shell]] energies. Excite a gas — with heat, or an [[Electric_current|electric discharge]] — and it emits only at its own characteristic wavelengths, producing bright [[Spectral_line|lines]] on a dark background. ## Absorption is the same picture inverted Pass continuous light through a cool gas instead and the same wavelengths go missing, leaving dark lines — the absorption spectrum. That symmetry is what allows a star's composition to be read from outside it, and it is how [[Helium]] was first identified. In the laboratory the technique underpins routine elemental analysis; in the [[Atmosphere_of_Earth|atmosphere]] it is how greenhouse absorption is measured. Because line intensity depends on temperature and [[Density]], a spectrum reports conditions as well as composition — a [[Sensor|measurement]] that needs no contact with the sample at all. **Reads with:** *Atomic Physics for Everyone (2025)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/atomic-physics-for-everyone-an-introduction-to-atomic-physics-quantum-mechanics-and-precision-spectroscopy-with-no-college-level-prerequisites) · [download](https://scholarworks.smith.edu/textbooks/9/). Seeded from the Thury Hydrodynamics Apex Spine book shelf. **On the spine:** [[Spectral_line]] · [[Electron_shell]] · [[Helium]] · [[Isotope]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Emission_spectrum) : [Wikitube](https://en.wikitube.io/wiki/Emission_spectrum) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Helium]], [[PORTAL_Physics]]. --- *Apex Spine wave · 2026-09-10 · stub · microsim layer deferred.*