Enumerate the common defects of vision, their causes and the possible methods of correcting them.

Defect of vision

Cause

Corrective measure

(i) Myopia

(i) Lengthening of eyeball from front to back or the lens is too curved.

(i) Using suitable concave lens

(ii) Hyperopia

(ii) Shortening of eyeball from front to back or the lens is too flat.

(ii) Using suitable convex lens

(iii) Astigmatism

(iii) Uneven curvature of the cornea

(iii) Using suitable cylindrical lenses

(iv) Presbyopia

(iv) Loss of flexibility of lens

(iv) Using suitable convex lens

(v) Cataract

(v) Lens turning opaque

(v) Surgery or use of convex lens or implantation of the plastic lens.

(vi) Colour blindness

(vi) Genetic defect

(vi) No control measure

(vii) Squint

(vii) Formation of cross-eye

(vii) Surgery and suitable exercise