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

Defect of vision	Cause	Corrective measure
Myopia	Lengthening of eye ball from front to back or the lens is too curved.	Using suitable concave lens
Hyperopia	Shortening of eye ball from front to back or the lens is too flat.	Using suitable convex lens
Astigmatism	Uneven curvature of the cornea	Using suitable cylindrical lenses
Presbyopia	Loss of flexibility of lens	Using suitable convex lens
Cataract	Lens turning opaque	Surgery or use of convex lens or implantation of plastic lens.
Colour blindness	Genetic defect	No control measure
Squint	Formation of cross-eye	Surgery and suitable exercise