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A person looking at a person wearing a shirt with a pattern comprising vertical and horizontal lines is able to see the vertical lines more distinctly than the horizontal ones. What is this defect due to ? How is much a defect of vision corrected ?

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This defect is called Astigmatism. It arises because curvature of the cornea plus eye lens refracting system is not the same in different planes. As vertical lines are seen distinctly, the curvature in the vertical plane is enough, but in the horizontal plane, curvature is insufficient.
This defect is removed by using a cylindrical lens with its axis along the vertical.
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