Orrection for refraction is done because (1) Of chromatic aberration (2) Of spherical aberration of the objective lens (3) The line of sight is horizontal and deviates from a level surface (4) The line of sight is refracted and deviates from its straight path?

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Explanation of Correction for Refraction

Correction for refraction is a process of correcting the deviation of the line of sight caused by the refraction of light in the atmosphere. This deviation can affect the accuracy of measurements, especially in surveying and astronomical observations. The correction is necessary to obtain the true position or direction of an object or point in space.

Causes of Refraction

Atmospheric Refraction: The Earth's atmosphere consists of layers of air with varying densities and temperatures. When light passes through these layers, it is refracted or bent due to the change in the refractive index of air. The amount of refraction depends on the angle of incidence and the density gradient of the atmosphere. The refraction is greater near the horizon than near the zenith, and it increases with the altitude of the object.

Instrumental Refraction: The lenses of optical instruments, such as telescopes, cameras, and binoculars, also cause refraction. The amount of refraction depends on the curvature and quality of the lenses, as well as the wavelength of light. The refraction can cause chromatic aberration, where different colors of light are focused at different distances, and spherical aberration, where the image is distorted due to the shape of the lens.

Correction Methods

There are different methods of correcting for refraction, depending on the application and the accuracy required.

Theoretical Refraction: This method uses mathematical models to calculate the amount of refraction based on the atmospheric conditions and the altitude of the object. The models include the International Standard Atmosphere (ISA) and the Saastamoinen model. The correction is applied as a function of the altitude or the zenith distance of the object. This method is suitable for high-precision measurements, such as geodetic surveys and astrometry.

Tabulated Refraction: This method uses precomputed tables of refraction values for different altitudes and atmospheric conditions. The tables are based on empirical observations and are specific to the location and time of observation. The correction is applied by looking up the value in the table that corresponds to the altitude of the object. This method is less accurate than theoretical refraction but is simpler and faster to use.

Measured Refraction: This method involves measuring the refraction directly using a theodolite or a zenith telescope. The correction is applied by subtracting the measured value from the observed altitude of the object. This method is suitable for low-precision surveys and observations.

In conclusion, correction for refraction is a necessary step in obtaining accurate measurements and observations in civil engineering, astronomy, and other fields that rely on optical instruments. The correction can be done using theoretical models, precomputed tables, or direct measurements, depending on the application and the level of accuracy required.

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