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Diagram showing displacement of the Sun's image at sunrise and sunset Comparison of inferior and superior mirages due to differing air refractive indices, n. Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. [1]
In atmospheric applications, refractivity is defined as N = n – 1, often rescaled as either [60] N = 10 6 (n – 1) [61] [62] or N = 10 8 (n – 1); [63] the multiplication factors are used because the refractive index for air, n deviates from unity by at most a few parts per ten thousand.
Atmospheric refraction influences the apparent position of astronomical and terrestrial objects, usually causing them to appear higher than they actually are. For this reason navigators, astronomers, and surveyors observe positions when these effects are minimal.
For example, air has a refractive index of 1.0002793 at atmospheric pressure, where there are about 2 × 10 25 molecules per cubic meter, and therefore the major constituent of the atmosphere, nitrogen, has a Rayleigh cross section of 5.1 × 10 −31 m 2 at a wavelength of 532 nm (green light). [14]
Comparison of inferior and superior mirages due to differing air refractive indices, n The sun appears slightly flattened when close to the horizon due to refraction in the atmosphere. The refractive index of air depends on the air density and thus vary with air temperature and pressure. Since the pressure is lower at higher altitudes, the ...
Refraction at interface Many materials have a well-characterized refractive index , but these indices often depend strongly upon the frequency of light, causing optical dispersion . Standard refractive index measurements are taken at the "yellow doublet" sodium D line , with a wavelength (λ) of 589 nanometers .
Looming of the Canadian coast as seen from Rochester, New York, on April 16, 1871. Looming is the most noticeable and most often observed of these refraction phenomena. It is an abnormally large refraction of the object that increases the apparent elevation of the distant objects and sometimes allows an observer to see objects that are located below the horizon under normal conditions.
When atmospheric refraction is considered, ray tracing becomes necessary (Kivalov 2007), and the absolute air mass integral becomes [7] = where is the index of refraction of air at the observer's elevation above sea level, is the index of refraction at elevation above sea level, = +, = + is the distance from the center of the Earth to a ...