CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 17
Due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens.
Answer
Presbyopia; corrected by bifocal lenses, with a concave upper part for distant vision and a convex lower part for near vision.
The defect is presbyopia: with age the power of accommodation of the eye decreases, the near point recedes, and it becomes difficult to see nearby objects comfortably and distinctly.
Such persons need a convex lens for near vision, and since with age many of them also find distant objects blurred, they commonly use bifocal lenses.
Structure: a common bifocal lens consists of both a concave and a convex lens; the upper portion is a concave lens and the lower portion is a convex lens.
Function: the upper concave portion facilitates distant vision, and the lower convex portion facilitates near vision, such as reading.
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