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 such a defect of vision corrected?

The person is not able to see horizontal lines clearly, whereas the vertical lines are distinctly visible. This problem arises due to problem in refracting mechanism of eye specially cornea and eye lens. This condition is called as astigmatism. The curvature of eye lens in the horizontal plane may be abnormal. Hence, sharp images of the vertical lines are formed on the retina, but horizontal lines appear blurred. This defect can be corrected by using cylindrical lenses.