DEFECTS OF VISION AND THEIR CORRECTION .

 DEFECTS OF VISION AND THEIR CORRECTION -

             THE EYE MAY GRADUALLY LOSE ITS ACCOMMODATION POWER. IN THESE CONDITION THE PERSON CAN NOT SEE THE OBJECT COMFORTABLY. THIS VISION BECOMES BLURRED BECAUSE OF THE  REFRACTIVE DEFECTS OF THE EYE .

 

THREE COMMON REFRACTIVE DEFECTS OF VISION :

    MYOPIA - 

                 IT IS ALSO CALLED A NEAR SIGHTEDNESS  . A PERSON SUFFERING FROM MYOPIA CAN SEE NEARBY OBJECT CLEARLY BUT ARE NOT ABLE TO SEE DISTANT OBJECTS DISTINCTLY. A PERSON SUFFERING FROM DEFECT HAS FAR POINT NEARER THAN INFINITY SUCH A PERSON MAY SEE DISTINCTLY UP TO DISTANCE OF FEW METERS IN A MYOPIC EYE , THE IMAGE OF A DISTANT OBJECT IS PRODUCED IN FRONT OF RETINA AND NOT EXACTLY AT THE RETINA ITSELF.

THIS DEFECT MAY ARISE BECAUSE OF : 

1) EXCESSIVE CURVATURE OF THE EYE LENS 

2) ELONGATION  OF THE EYE BALL 

                  THIS DEFECT CAN BE CORRECTED WITH THE HELP OF CONCAVE LENS OF SUITABLE POWER. 

HYPERMETROPIA -                                                                                                                                                                    IT IS ALSO KNOWN AS FAR SIGHTEDNESS. A PERSON WITH HYPERMETROPIA  CAN SEE DISTANT OBJECT CLEARLY BUT CAN NOT SEE NEARBY  OBJECT CLEARLY . THE NEAR POINT , FOR THE PERSON, IS VERY FAR AWAY FROM THE NORMAL NEAR  POINT ( 25 CM )  THIS IS DUE TO THE LIGHT RAYS FROM A NEAR BY OBJECT FOCUSED AT A POINT BEHIND .

THIS DEFECT ARISES EITHER DUE TO :

1)  THE FOCAL LENGTH OF THE EYE LENS IS TOO LONG . 

2)  THE EYE BALL HAS BECOME TOO SMALL.

                               THIS DEFECT CAN BE CORRECTED WITH THE  HELP OF A CONVEX LENS OF APPROPRIATE POWER . 

PRESBYOPIA - 

           THE POWER OF ACCOMMODATION OF THE EYE GENERALLY DECREASES WITH AGEING. FOR MANY PEOPLE THE NEAR POINT RECEDES AWAY GRAUDALLY . IT IS DIFFICULT TO SEE  CLOSE BY OBJECTS DISTINCTLY WITHOUT CORRECTIVE EYEGLASSES . THIS DEFECT IS KNOWN AS PRESBYOPIA. IT ARISE DUE TO THE DIMINISHING FLEXIBILITY  OF THE EYE LENS AND THE GRADUAL WEAKENING OF THE CILIARY  MUSCLE . A COMMON TYPE OF BI-FOCAL LENSES HAVE BOTH CONCAVE AND CONVEX LENS . THE UPPER HAVE A CONCAVE LENS . IT CORRECTED THE DISTANT VISION . THE LOWER PART IS A CONVEX LENS. IT CORRECTS THE NEAR VISION .