Defects of vision, symptoms & remedy (correction)
Defects of Vision
The major defects of vision are :
- Nearsightedness or myopia.
- Farsightedness or hypermetropia.
Nearsightedness or myopia
Symptoms : Eye cannot see clearly beyond a certain distance. It means that the far point of the defective eye has shifted from infinity to a finite distance ahead.
Reasons : It is so because the image of distant objects is formed in front of the retina. It is shown in fig.
(i) The lens may be thicker (more converging) that the normal eye lens.
(ii) The eye ball may be elongated,
Correction : The extra converging power of eye lens is compensated by using a concave (diverging) lens of proper power (focal length) as shown in fig.
Calculation : Let distance of far point F’ from eye = x. Then for lens to be used, u = ∞, v = –x, f = ?
From lens formula,
or f = –x
The lens used must have focal length equal to the distance of the far point from the eye (–ve sign means concave lens).
Farsightedness or hypermetropia
Symptoms : With this defect eye cannot see clearly within a certain distance. It means that the near point of the defective eye has shifted from 25 cm to some more distance behind (away).
Reason : It is so because the image of near objects is formed behind the retina. It is shown in fig.
(i) The eye lens may be thinner (less converging) than the normal eye lens.
(ii) The eye ball may be oval distance between lens and retina becomes less than that for normal eye.
Correction : The deficiency in converging power of eye lens is compensated by using a convex (Converging) lens of proper power (focal length) as shown in fig.
Calculation : Let distance of near point N’ from eye = y. Then, for lens to be used
u = –D, v = –y, f = ?
From lens formula,
f = (Which is positive Q y > D)
This is required expression for the focal length of the convex lens to be used.
Presbyopic : 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 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 facilitates distant vision. The lower part is a convex lens. It facilitates near vision. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
Astigmation : A person suffering from this defect cannot simultaneously focus on both horizontal and vertical lines of a wire gauze.
This defect arises due to the fact that the cornea is not perfectly spherical
This defect can be corrected by using cylindrical lens