Human Eye

OPTICAL INSTRUMENTS


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CATARACT PRESBYOPIA AND ASTIGMATISM


1) Cataract

       Cataract is usually found in old persons. Cataract causes the loss of eye sight due to the formation of an opaque membrane over the eye lens.

NormalEye            Eye Suffering from cataract

                    Normal eye                       Eye Suffering from cataract

Correction of Cataract

       This defect cannot be corrected by using any type of spectacles. The only method for its correction is the removal of opaque lens by cataract surgery.

2) Presbyopia or Old Age Longsightedness

       Due to Presbyopia an old person cannot see nearby objects clearly. That is why he feels difficulty in reading and writing. This defect occurs due to the loss of elasticity of eye lens and gradual weakening of cilliary muscles.

Presbyopia


Correction of Presbyopia

       This defect is also corrected by using spectacles having suitable convex lens.

3) Astigmatism

       Due to astigmatism blurred images are formed due to irregularities in the curvature of cornea. Due to this defect, the light rays coming from an object are focused at different points.

Correction of Astigmatism

       This defect is corrected by using cylindrical lenses.



Test Your Understanding and Answer These Questions:




  1. What is cataract?

  2. How can cataract be cured?

  3. What is presbyopia?

  4. How can presbyopia be corrected?

  5. What is astigmatism?

  6. How can astigmatism be corrected?





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