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the human eye and the colourful world — Science STD 10 — Question

Gujarat BoardEnglish MediumSTD 10Sciencethe human eye and the colourful world3 Marks
Question
Write a short note on Presbyopia.
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Answer

  • Presbyopia is the defect of vision in which aged people find it difficult to see the nearby objects comfortably and distinctly without spectacles. OR
  • Presbyopia is that defect of human eye due to which an old person cannot read and write comfortably.
  • The power of accommodation of the eye usually decreases with ageing.
  • For most people, the near point gradually receds away.
  • They find difficult to see nearby objects comfortably and distinctly without corrective eyeglasses.
  • Sometimes a person may also find it difficult to see distant objects clearly without corrective eyeglasses.
  • In this situation, the far point is shifted from infinity towards the person.
  • Thus, sometimes, a person may suffer from both myopia and hypermetropia.
  • Causes of presbyopia:
  • This defect arises due to gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
  • Hence the main cause of presbyopia is only increase in focal length of eye lens whereas the eyeball in presbyopia has normal length.
  • Correction of the defect:
  • This defect is corrected using a convex lens of suitable power.
  • A person suffering from both myopia and hypermetropia has to use spectacles of bi-focal lenses, i.e., concave and convex lenses.
  • Normally, the upper portion consists of a concave lens which facilitates distant vision while the lower part is a convex lens which facilitates near vision.

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