Class 10 Physics Human Eye and Colourful World Defects of Vision and their Correction

Defects of  Vision and their Correction

 

Three common refractive defects of the eye:

  • Myopia or Near-sightedness
  • Hypermetropia or Farsightedness
  • Presbyopia

 

 Myopia

  • Near-sightedness : A person with Myopia can see nearby objects clearly
  • A person with myopia cannot see faraway objects clearly.
  • The far point for the myopic eye is nearer than infinity
  • Occurs due to
    • Excessive curvature of the eye lens
    • Elongation of eyeball
  • The image of a distance object is formed in front of the retina and not on the retina
  • Defected is corrected by using Concave lenses such that the lens will bring the image back on to the retina.

 

 

 

 

 Hypermetropia

  • Far sightedness : A person with Hypermetropia can see faraway objects clearly.
  • A person with Hypermetropia cannot see nearby objects clearly.
  • The near point of the eye is more than 25cm
    • This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.
    • Focal length of the eye lens is too long
    • Eyeball has become too small. 
  • The image of a distance object is formed in behind the retina and not on the retina
  • Defected is corrected by using Convex lenses such that the lens will bring the image back on to the retina.

Presbyopia

  • The power of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.
  • The near point moves away.
  • Spectacles with convex lenses are recommended

 

 

Myopia and Hypermetropia

  • Sometimes a person may suffer from both near sightedness and far-sightedness.
  • Such people are advised to use bifocal lenses.
  • Bifocal lenses consists of concave on the upper portion and convex on the lower portion.
  • Concave supports distinct vision and convex supports near vision

Refractive eye defects can also be corrected using contact lenses or through specific surgical procedures.

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