Defects of Vision – 10th Board

Myopia is a disease of eye. The eye is unable to see distant objects clearly but can see nearby objects well. The image of distant object forms before the retina.

Reasons of myopia include elongation of eyeball and increase in curvature of eye-lens i.e. decrease in its focal length.

Use of concave lens of suitable power corrects this defect.

Hypermetropia is an eye disease. In this case, the eye is able to see the distant objects but cannot see the nearby objects clearly. The formation of the image takes place beyond the retina.

There are mainly two reasons of hypermetropia. First, the eyeball becomes small in length. Second, the curvature of the eye lens decreases i.e. its focal length increases.

A convex lens of suitable power corrects Hypermetropia.

Presbyopia is an eye disease of elderlies in which they are unable to see nearby objects. Ciliary muscles weaken with ageing. Their capability to contract decreases. Thus, eye-lens cannot bend as required for focusing nearby objects.

Use of convex lens of suitable power corrects hypermetropia. It is similar to correction of hypermetropia.

Yes. An eye can suffer with both the diseases, myopia and hypermetropia. In such cases, eyeglasses of bifocal lengths are used.

Bifocal lenses consist of convex as well as concave lens. Its upper portion is concave lens to see distant objects while lower portion is convex lens to see nearby objects.

Yes, it is possible. Contact lenses and surgeries are new techniques to cure such diseases.

Cataract is an eye disease in which eye-lens turns milky and cloudy. It causes partial or complete loss of vision. The disease is common among old people.

Replacement of eye-lens through surgery is the cure of the cataract.