2. The degree to which the apparent position of an astronomical object is distorted by the redirection of its light as it passes through the earth's atmosphere.
3. In ophthalmology, the ability of the eye to change the direction of light in order to focus it on the retina.
4. Etymology: from Late Latin refractionem, refractio, "a breaking up", from Latin refractus, past participle of refringere, "to break up", from re-, "back" + frangere, "to break".
2. An apparent upward displacement of celestial objects relative to the horizon as light from them is bent toward the vertical by the decreasing density with altitude of the earth's atmosphere: Atmospheric refraction is greatest for objects on the horizon and negligible at elevations higher than about 45 degrees.
The angular difference between the apparent zenith distance of a celestial body and its true zenith distance is produced by refraction effects as the light from the body penetrates the atmosphere.
Any refraction caused by the atmosphere's normal decrease in density with height.
Near surfaces on the earth, those within a few meters or so, are usually dominated by temperature gradients.