Human Eye, due to wear and tear or due to accidents develops defects. These defects are called defects of vision.
Here we cover four defects of vision
1. Nearsightedness, also called myopia is a common name for impaired vision in which a person sees near objects clearly while distant objects appear blurred.
2. A person with myopia:
(i) can see nearby objects clearly
(ii) but cannot see distant objects distinctly.
3. A person with this defect has the far point nearer than infinity. Such a person may see clearly up to a distance of a few meters. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
4. Causes: This defect may arise due to
(i) an excessive curvature of the eye lens, or
(ii) elongation of the eyeball.
5. Correction: 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.
1. Farsightedness, also called hypermetropia, common name for a defect in vision in which a person sees near objects with blurred vision, while distant objects appear in sharp focus.
2. A person with hypermetropia
(i) can see distant objects clearly;
(ii) but cannot see nearby objects distinctly.
3. The near point, for the person, is farther away from the normal near point (25 cm). Such a person must keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a close by the object are focussed at a point behind the retina.
4. Causes: This defect arises either because:
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small so that light rays from the nearby object cannot be brought to focus on the retina to give a distinct image.
5. Correction: 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.
1. Presbyopia, progressive form of farsightedness that affects most people by their early 60s. The power of accommodation of the eye usually decreases with aging. For most people, the near point gradually recedes away.
2. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses.
3. Causes: It arises due to the
(i) gradual weakening of the ciliary muscles; and
(ii) diminishing flexibility of the eye lens.
4. Correction: Simple reading eyeglasses with convex lenses correct most cases of presbyopia.
5. Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses. In the bi-focal lens, the upper portion of the bi-focal lens is a concave lens, used for distant vision. The lower part of the bi-focal lens is a convex lens, used for reading purposes.
6. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
1. Astigmatism, a defect in the outer curvature of the surface of the eye that causes distorted vision. In astigmatism, a person cannot simultaneously focus on both horizontal and vertical lines.
2. Causes: This defect is usually due to
(i) the cornea that is not perfectly spherical. Consequently, it has different curvatures in different directions in vertical and horizontal planes.
(ii) This results in objects in one direction being well-focused, while those in a perpendicular direction not well-focused.
3. Correction: This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea.
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