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Defects of vision and their corrections:
There are three common defects of vision. They are:
1) Myopia (Short-sightedness or Near-sightedness)
2) Hypermetropia (Long-sightedness or Far-sightedness)
The defect of an eye in which it cannot see the distant objects clearly is called myopia. A person with myopia can see nearby objects clearly. Myopia is caused due to:
- High converging power of lens
- Eye-ball being too long
Due to high converging of the eye-lens the image is formed in front of the retina and a person cannot see clearly the distant objects. In another case, if the eye- ball is too long than the retina is at larger distance from the eye-lens. In this case also the image is formed in front of the retina even though the eye-lens has correct converging power.
Myopia or short-sightedness can be corrected by wearing spectacles containing concave lens. This is because when a concave lens of suitable power is used for the myopic eye then the concave lens first diverge the parallel rays of light coming from distant object. Therefore, first a virtual image is formed at the far point of the myopic eye. Now since the rays of light appear to be coming from eye’s far point, they are easily focussed by the eye-lens and image is formed on retina. Concave lens is used for myopic eye so as to decrease the converging power of the eye-lens.
Formula for calculating power of concave lens to correct myopia is:
1/image distance (v)-1/object distance (u) = 1/focal length (f)
Hypermetropia or long-sightedness is a defect of an eye where a person cannot see nearby objects clearly. The near-point of hypermetropic eye is more than 25 cm away. This defect of eye is caused due to:
- Low converging power of eye-lens
- Eye-ball being too short
In case of hypermetropia the image of an object is formed behind the retina and therefore, a person cannot see clearly nearby objects.
The near-point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be corrected by putting a convex lens in front of the eye. This is because when a convex lens of suitable power is placed in front of the hypermetropic eyes, then the convex lens first converge the diverging rays of light coming from a nearby object at the near point of the eye at which the virtual image of the nearby object is formed. Since the light rays now appear to be coming from the eye’s near point, the eye-lens can easily focus and form the image on retina. Convex lens is used for hypermetropia so as to increase the converging power of the eye-lens.
Correction of Hypermetropia: The convex lens forms a virtual image of the object (lying at normal near point N) at the near point N’ of this eye.
Formula for calculating power of convex lens to correct hypermetropia is:
1/v – 1/u = 1/f
In this formula, object distance that is u, is normal near point of the eye (25 cm).
This defect of vision usually happens in old age when ciliary muscles become weak and can no longer adjust the eye-lens. The muscles become inflexible in this condition and cannot see nearby objects clearly.
The near-point of an old person having presbyopia is much more than 25 cm. Presbyopia can be corrects by wearing spectacles having convex lens.
Another point to be noted is that a person can have both myopia and hypermetropia. In such a condition, spectacles having bifocal lens are worn. The upper part of bifocal lens is concave and lower part consists of convex lens.
A yet another defect of the eye which usually comes in old age is the cataract. The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision. It develops when the eye-lens of a person becomes cloudy due to the formation of a membrane over it. It decreases the vision of the eye gradually and can lead to total loss of vision of the eye. It can be restored after getting surgery .The opaque lens is removed and artificial lens is inserted in its place via operation. This defect cannot be corrected by any type of spectacle lenses.
Why do we have two eyes for vision?
- Two eyes give wider field view of 180 degree.
- Two eyes help judge the distance of an object more accurately.
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