Human Eye- Part 5

Note:- The whole purpose of using concave lens is to decreases the converging power of the eye lens and helps in forming the image of distant object on the retina of the myopic eye. 

(ii) Hypermetropia- The defect of eye in which a person can see distant objects clearly but can’t see the nearby objects clearly is called hypermetropia. The near point of hypermetropic eye is more than 25cm away. 

     Hypermetropia is caused due to:- 

 * Low converging power of eye lens 

 * Eyeball being too short 

 -> Since a convex lens has the ability to converge incoming rays. So, Hypermetropia is corrected by using convex lenses. when a convex lens of suitable power is placed in front of hypermetropic eye then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this, the convex lens forms a virtual image of the nearby object at the near point of the hypermetropic eye. 

Note:- The whole purpose of using a convex lens is to increase the converging power of the eye lens. 

(iii) Presbyopia- In old age, when ciliary muscles become weak and the eye lens become inflexible or rigid, then the eyes loses its power of accommodation and due to this, an old person can’t see the nearby objects clearly. So, the defect of vision which an old person can’t see the nearby object clearly is called presbyopia. 

     Presbyopia is caused due to:- 

      * decrease the flexibility of eye lens 

      * gradual weakening of ciliary muscles 

Correction -> Presbyopia is corrected in the same way as hypermetropia by using spectacles having convex lenses. 

 * Difference between the cause of hypermetropia and presbyopia is- The cause of Hypermetropia is decrease in length of eyeball or increase In focal length of eye lens. 

   But, the cause of Presbyopia is only increase in focal length of eye lens. In Presbyopia, the eyeball has normal length. 

  Note:- In Presbyopia, a person can also suffer from both myopia and hypermetropia. 

Correction-> A person suffering from both myopia as well as hypermetropia uses a spectacles having bifocal lenses in which upper part consists of concave lens i.e. used for distant vision and the lower part consists of convex lens i.e. used for reading purpose or nearby objects. 

Other defects of vision are:- 

* Astigmatism- It is a kind of defect in human eye due to which a person cannot see (focus) simultaneously horizontal and vertical lines both. 

 Correction- By using a cylindrical lens 

* Colour Blindness- When a person is not able to distinguish between the different colours due to its defective cone cells then that defect is known as Colour Blindness.

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