Some people can not see clearly objects due to loss of power of accommodation of eye. Such problem may be due to –
1. weakening of ciliary muscles
2. change in size of eyeball
3. Irregularity on the surface of cornea
4. Formation of membrane over the eye lens
The main three common defects of vision are following-
- Myopia or near-sightedness
In this defect, person can see near objects clearly but he is unable to see far objects clearly. Therefore, it is called near-sightedness. In this case, the near point of the eye is at normal but the far point is not at infinity. The image formed of far objects by the eye lens is in front of the cornea and not on the cornea. This is shown in fig. below.
A concave lens of suitable power corrects this defect. It focuses the light rays from far object at the far point of eye. Therefore, the image of the object is formed at retina by the eye lens.
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2. Hypermetropia or far-sightedness –
In this defect, person is unable to see near objects clearly however he can see far objects. Therefore, it is called far-sightedness.The near point for the person is farther away from the normal near point. Normal near point is at 25 cm. The image of object at normal near point is made by the eye lens behind the retina as shown in fig.
This defect arises because
(1) the focal length of the eye is too long or
(2) the eyeball has become too small.
This defect is corrected by a convex lens of suitable focal length. The convex lens makes the image of object placed at normal near point at the near point of the eye as shown in fig. The eye lens focuses the image to retina. Thus eyeglasses with converging lens provide power to focus image at retina.
3. Presbyopia –
The power of accommodation of the eye decreases with age. For old people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eyeglasses. This defect is called presbyopia. It arises due to 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 bi-focal lenses. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It is for facilitating distant vision. The lower part is a convex lens. It facilitates near vision.
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