Spectral line

A narrow feature at a particular frequency or wavelength in a spectrum, produced when matter emits or absorbs radiation at that energy.

Emission spectrum: A spectrum showing radiation emitted by a source, often as bright lines against a darker background. Its bright features arise when excited matter emits photons at characteristic energies.
Electromagnetic spectrum: The full range of electromagnetic radiation, ordered by frequency or wavelength. Spectral lines are localized features within this broader distribution of radiation.
Spectroscopy: The study of how matter interacts with electromagnetic radiation as a function of wavelength or frequency. Spectroscopy uses line patterns to identify substances and infer physical conditions.
Hydrogen-alpha: The red spectral line of hydrogen produced by the transition from the third to the second principal energy level. This prominent line traces ionized hydrogen in stars, nebulae, and the Sun.
Absorption spectrum: A spectrum with reduced intensity at wavelengths absorbed by matter along the line of sight. Its dark features mark energies removed from a continuous or broader source spectrum.
Photon: A quantum of electromagnetic radiation carrying energy and momentum. A photon emitted or absorbed in a transition carries the line's energy.
Doppler spectroscopy: A method that measures motion by detecting frequency shifts in spectral features. Shifts in line centers reveal radial motion of emitting or absorbing matter.
Fraunhofer lines: Dark absorption lines in the solar spectrum, catalogued in the early nineteenth century. Their pattern established that elements in the Sun imprint identifiable lines.
Atomic transition: A change in an atom's energy state involving absorption or emission of a photon. The energy difference between states sets a line's characteristic frequency.
Planck relation: The relation stating that a photon's energy equals Planck's constant times its frequency. It converts a line's measured frequency into the energy of its transition.

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