The human eye is called defected eye which may be due to old age, biological change or any other reason. It can be corrected then the eye is called corrected eye.
Following are the importance defects of vision—
(1) Hypermetropia—(or longsightedness or for sightedness), The human eye is said to the hypermetropic if it can seen the far objects clearly but cannot see near objects clearly and distinctly.
Cause—
(a) It is due to increase in focal length of eye lens.
(b) Decrease in the size of eye ball.
Correction-
It is corrected by convex lens.
2. Myopia—The human eye is said to be myopia if it can see the near object clearly and distinctly but can not see the far objects clearly and distinctly.
Cause—The defect arises due to elongation of the eye ball or excessive curvature of cornea.
Correction—It is corrected by concave lens.
3. Presbyopia—(or old sight) The human eye is said to be presbyopic if it can not see near objects as well as far objects clearly and distinctly.
Cause—It is due to the weaken strength of cilliary muscles.
Correction—It is corrected by “bifocal lens”. Upper part is concave lens & power part is convex lens.
4. Astigmation—The human eye is said to be astimatic if horizontal lines and verticle can not be distinguished at the same time.
Cause—The defect arises due to different curvature in different direction of cornea.
Correction—It is corrected by cylindrical lens.
