Ncert Exercises & Solutions

9.27: A person looking at a cloth with a pattern consisting of 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 such a defect of vision corrected?

Hint: Recall defects of the human eye.

Step: Find the cause of astigmatism.
The person can not see the horizontal lines distinctly but he can see the vertical lines distinctly. It is due to the refracting system of the eye not working in the same way for different planes. This is known as astigmatism. The curvature of the eye is sufficient for the vertical plane but not sufficient for the horizontal plane. Hence, the formation of sharp images from the vertical lines is possible on the retina and the horizontal lines appear blurred. With the help of cylindrical lenses, this defect can be corrected.

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