Defects in Eye:

a) Myopia or short sightedness.

b) Hypermetropia or long sightedness.

c) Astigmatism.

d) Presbyopia.

 

 

 

a) Myopia or short sightedness:

1) In this case, light  coming from a distant object arrives our eye in such a way that after refraction the eye lens converses light at a point in front of the retina



2) So the person can not see the object which are at very large distance. But he can see the objects which are closer to the observer.


3) But using an appropriate focal length of concave lens in between the eye and the object, the incoming light can be focused on the retina and myopia defect can be solved.

 

 

b) Hypermetropia:

1) In this case the focal length of the eye lens decreases. So the light coming from objects which are closer to the observer, eye lens focuses the incoming light behind the retina.


2) So observer can see the object which are far from the eye, but can not see the objects which are closer to the eye.

3) To solve this problem, an appropriate focal length of convex lens is placed in between the observer eye & the object.  


 

c) Presbyopia:


1) As age increases, our muscle which holds our eye lens become weak. The closest distance a person can see clearly is also increases.


2) For a healthy eye this closest distance for clear vision is 25 cm.


3) But for elder person this distance increases sometime to 100cm, 150cm or more like this.


4) And person can not see clearly when the object is closer to eye.


5) This defect is called Presbyopia.


6) A proper convex lens is used to solve this problem.



 

d) Astigmatism:

1) When the shape or curvature of the cornea is not normal and irregular then the person can not distinguish an object properly a sharp image does not formed in his retina.


2) To solve this Astigmatism problem, a cylindrical lens of suitable curvature is used and placed in between the object and the eye.