# Refraction Definitions for Land Surveyors

refraction—The bending of light rays in passing from one transparent medium into a medium which has a different index of refraction. The angle of refraction is the angle which the refracted ray makes with the normal to the surface separating the two media.

refraction, astronomical—The refraction by the Earth’s atmosphere of light from a source outside the atmosphere. Light from a celestial body. such as a star or planet. passes entirely through the atmosphere in reaching the Earth, following a curved path which is concave to the surface of the Earth. The angle between the direction of a ray where it enters the atmosphere and its direction at the point of observation is the astronomical refraction of the my, or, its refraction. The magnitude of an astronomical refraction is greater when the observed body is near the horizon; with slight irregularities, it decreases to a minimum near the zenith, being zero when the path of the ray is normal to the surfaces of the atmospheric layers. Astronomical refraction must be considered in many problems of astronomy. surveying. and navigation. Also known as “celestial refraction.”

refraction, horizontal—The lateral effect of terrestrial refraction which affects the observed values of horizontal directions. See also refraction, terrestrial.

refraction, index of—The sine of the angle of incidence divided by the sine of the angle of refraction equals a constant when one of the media is air. The index of refraction can also be explained as the ratio of the velocity of light in one medium to that in another. ‘The indices of glass range front 1.40 to 1.80. See also refraction, Snell’s law of

refraction, Snell’s law of—When a wave crosses a boundary, the wave normally changes direction in such a manner that the sine of the angle of incidence between wave normal and boundary normal divided by the velocity in the first medium equals the sine of the angle of refraction divided by the velocity in the second medium.

refraction, terrestrial—The refraction by the Earth’s atmosphere of light from a terrestrial source. The path of light from a terrestrial source is usually far from horizontal; it passes through only the lower strata of the atmosphere and stiffen refraction throughout its entire length. Because the arrangement of the air strata is not exactly symmetrical in form and density, the path of a ray of light through the atmosphere is not a smooth curve in a vertical plane. Its curvature is greatest, however, in or dose to a vertical plane, when ‘ magnitude is of considerable importance in observations which are referred to the zenith or to the plane of the horizon, as are some vertical angles .- spirit levels. Terrestrial refraction usually makes the apparent altitude of an object greater than its true altitude, although under special atmospheric conditions this effect may be reversed. In triangulation, a station which is normally just below the apparent horizon may. under certain atmospheric conditions and because of refraction, become visible for a short period of time and be observable. A line which out be observed only with the .ml of refraction is called “refraction line.” Refraction in a direction which is approximately horizontal is lateral refraction;” this can, under conditions, be of such magnitude as to affect seriously the observed values of horizontal directions. A careful reconnaissance is required, especially when surveying urban areas, to prevent the inclusion of lines so affected in a survey net. In some publications terrestrial refraction has been referred to as “atmospheric refraction.” See also refraction, horizontal.

refraction, terrestrial (angle of)— The angle of refraction, or the refraction, of a survey line is the angle at the point of observation between the true direction of the observed object and the direction as shown by light coming from it (direction of object as seen).

refraction, vertical—That component of refraction, in the Earth’s atmosphere, which occurs in a vertical plane. Two kinds of vertical refraction affect surveying: that caused by the curvature of the atmospheric layers of drifting densities, and that caused by temperature gradients near the ground.

refraction line—See refraction, horizontal; refraction, terrestrial.

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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