Atmospheric refraction

Atmospheric refraction is the bending of light as it passes through air layers with different densities and refractive indices. It shifts the apparent positions of celestial objects and alters the paths of light near Earth’s surface.

Refractive index: A dimensionless measure of how much a medium slows light relative to its speed in vacuum. Differences in air’s refractive index make light change direction across atmospheric layers.
Atmospheric refraction correction: An adjustment to an observed direction or position that accounts for atmospheric bending of light. Surveyors and astronomers use corrections to estimate unrefracted directions.
Sunset: The daily event in which the Sun disappears below the local horizon. Refraction keeps the Sun’s image visible briefly after its geometric position passes below the horizon.
Atmospheric dispersion: The wavelength-dependent separation of light caused by the atmosphere’s refractive index varying with wavelength. It accompanies refraction but explains color separation rather than bending alone.
Snell's law: An equation relating the angles of incidence and refraction to the refractive indices of two media. It describes the directional change at each boundary in a layered-atmosphere model.
Geodetic surveying: The measurement of Earth’s shape, size, and precise surface positions. Long sight lines require refraction estimates to avoid errors in elevation and distance.
Green flash: A brief green appearance at the upper edge of the Sun near sunrise or sunset. Atmospheric refraction and wavelength-dependent dispersion help separate the Sun’s colored images.
Parallax: The apparent change in an object's position when viewed from different locations. Parallax shifts apparent position through viewpoint change, not bending within air.
Fermat's principle: A principle stating that light follows a path for which travel time is stationary. It explains curved atmospheric ray paths through variation in light’s speed.
Radio occultation: A remote-sensing method that analyzes signals changing as a spacecraft passes behind a planetary atmosphere. Atmospheric bending of radio signals reveals profiles of temperature and density.

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