Human Eye and Colourful World: Vision Defects of Human Eye

Many people develop vision defects in their eyes such as myopia, hypermetropia and presbyopia. These vision defects are easily corrected by using suitable lenses. Cataract is a vision defect which is rectified by surgery.

These notes are based on the chapter Human Eye and Colourful World from class 10 science NCERT book and CBSE syllabus.

Myopia
Hypermetropia
Presbyopia
Cataract

Myopia

Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to

excessive curvature of the eye lens, or
elongation of the eyeball.

Due to this, the far point of myopic eye is less than infinity. As a result, when an object is at infinity, its image is formed before retina instead of on retina. So, a person with myopic eye sees blurred image of a distant object.

Correction of Myopia

This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.

Hypermetropia

Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This defect arises either because

the focal length of the eye lens is too long, or
the eyeball has become too small.

The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a nearby object are focused at a point behind the retina.

Correction of Hypermetropia

This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.

Presbyopia

The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.

Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

Cataract

In old age the cornea becomes cloudy or foggy because of deposition of debris under the cornea. This reduces the vision in old age. Old age, diabetes, hypertension and pollution increase the risk of cornea.

Cataract can be cured by eye surgery. In this operation, doctor clears the debris from under the cornea which helps in restoring the vision. Now-a-days, artificial lens is also transplanted during cataract surgery. This is called Intra Ocular Lens Transplantation.