Explain briefly how the following defects of the eye are caused and how can they be corrected?a)Astigmatism b) Presbyopia
Astigmatism is an eye condition with blurred vision as its main symptom. The front surface of the eye (cornea) of a person with astigmatism is not curved properly - the curve is irregular - usually one half is flatter than the other - sometimes one area is steeper than it should be.
When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.
Way of correction:
Astigmatism can be corrected with glasses, contact lenses, refractive laser surgery and Implantable contact lenses.
Astigmatism can be corrected by soft contact lenses as well as hard contact lenses. Contact lenses for astigmatism are known as toric lenses .
Presbyopia is a common defect of vision, which generally occurs at old age. Presbyopia is the defect of eye in which a person cannot see further as well as nearer objects clearly. A presbyopic eye has its near point greater than 25 cm and it gradually increases as the eye becomes older.
Presbyopia is caused by the
1. weakening of the ciliary muscles
2. reduction in the flexibility of the eye lens
A person with presbyopia cannot read letters without spectacles. It may also happen that a person suffers from both myopia and hypermetropia. This type of defect can be corrected by using bi-focal lenses. A bifocal lens consists of both convex lens (to correct hypermetropia) and concave lens (to correct myopia).
It is a common misconception among people that the use of spectacles “cures” the defects of vision. However, this is not true as spectacles only “restore” the defects of vision to the normal value.
Ideally, an eyeball is shaped like a perfectly round ball. Light comes into it and bends evenly, which gives you a clear view. But if your eye is shaped more like a football, light gets bent more in one direction than another. That means only part of an object is in focus. Things at a distance may look blurry and wavy.
Astigmatism is fairly easy for an eye doctor to fix with glasses, contacts, or surgery.