Defects Of Vision
Inspite of all precautions and proactive action, our eyes may develop some defects due to various reasons. Some of the common optical defects of the eye are:
1. Myopia or short sightedness
2. Hypermetropia or long sightedness
3. Presbyopia
4. Astigmatism
5. Phorias
We discuss them here in some detail.
a. Myopia or short sightedness: Myopia or short sightedness is that defect of human eye by virtue of which, the eye can see clearly the objects lying near it, but the far objects cannot be seen distinctly i.e. objects lying beyond a particular distance cannot be seen by the eye. In other words, for a myopic eye, the far point shifts towards the eye. It is no longer at infinity. It is at F.
In fig. 1 parallel rays from infinity are focused on the retina by the normal eye. In fig. the myopic eye focused the || rays from infinity at P in front of the retina. However, the defective eye focuses rays from a point F on the retina. Thus F is the far point of the myopic eye. It cannot see beyond F.
The two possible causes of this defect are:
(i) Increase in size of the eye of the eye ball i.e. distance of retina from the eye lens increases.
(ii) Decrease in focal length of the eye lens, when the eye lens, when the eye is fully relaxed.
To correct a myopic eye of this defect, the person has to use spectacles with a concave lens of suitable focal length. In fact, parallel rays of light from infinity, after refractive through the concave lens, should appear to come from F, the far point of the defective eye. This is shown in fig. 1.
Let x be the distance of far point of the myopic eye.
And ƒ be the focal length of the concave lens to be used.
Now, for concave lens, u = ∞, v = -x
As 1/ƒ = (1/v – 1/u)
Or 1/ƒ = (- 1/x – 1/∞)
Or, ƒ = -x
Hence focal length of the concave lens put in front of the eye should be equal to distance of far point of the defective eye.
b. Hypermetropia or long sightedness
Hypermetropia or long sightedness is that defect of human eye, by virtue of which the eye can see clearly the far off objects, but the nearly objects cannot be seen clearly i.e. objects which are lying nearer than a certain distance cannot be seen by the eye. In other words, for a hypermetropic eye, the near point shifts away from the eye. It is no longer at 25 cm, the least distance vision.
In fig. 2 rays from the near point (N) are focused on the retina by the normal eye. N is at the least distance of distinct vision (25 cm).
In fig. 2 the hypermetropic eye focuses the rays from N at P’, a point beyond the retina. However, the defective eye focuses rays from another point N’ on the retina. Thus the near point of hypermetropic eye has shifted away from N to M’.
The two possible causes of this defect are:
(i) Contraction in the size of the eye ball i.e. distance of retina from the eye lens has decreased.
(ii) Increase in the focal length of the eye lens, when the eye if fully relaxed.
To correct a hypermetropic eye of this defect, the person has to use spectacles with a convex lens of suitable focal length. In fact, the rays starting from N, after refraction through the convex lens, from N, after refraction through the convex lens, should appear to came from N’ so that they are focused on the retina, fig. 2
Suppose x’ is the distance of near point N’ of the defective eye.
d is the least distance of distinct vision (of normal eye).
ƒ is focal length of convex lens to be used.
For the correcting lens,
u = - d, v = -x’
As 1/ƒ = (1/v – 1/u)
Therefore, 1/ƒ = [1/ (-x) + 1/d]
= (- d + x’/x’d)
Or ƒ = (x’d/x’ – d)
As x’. d, ƒ is +, the correcting lens must be convex.
c. Presbyopia (old sight): With increasing age, the ciliary muscles holding the eye lens weaken and the lens loses some of its elasticity. Therefore, power of accommodation of eye decreases with age. This defect is called Presbyopia. To remove this defect, converging spectacle lenses are employed (as in the case of Hypermetropia). The person needs to wear these specs for reading or similar close work.
d. Astigmatism: to a normal eye, all the lines look equally black, but an astigmatic eye will find variation in the intensity of different lines. This defect arises when cornea has different curvature in different directions.
Astigmatism can be corrected by suing a cylindrical lens of suitable radius curvature, and suitable axis.
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