The ability to see is called vision. It is also called eyesight. Sometimes the eye of a person cannot focus the image of an object on the retina properly. In such cases the vision of a person becomes blurred and he cannot see either the distant objects or nearby objects clearly and comfortably. The person is said to have a defect of vision.
There four common defects of vision .
1. Myopia ( short-sightedness or near-sightedness ) :
Symptoms :
Myopia is that defect of vision due to which a person cannot see the distant objects clearly.
The far point of an eye suffering from myopia is less than infinity.
Reasons :
In a eye suffering from myopia , the cillilary muscles attached to the eye-lens donot relax sufficiently to make the eye-lens thinner to reduce its converging power. The image of a distant object is formed in front of the retina.
Causes :
Due to high converging power of eye-lens ( because of its short focal length ) Due to elongated eye-ball being too long.
Correction :
Myopia is corrected by using spectacles containing concave lenses ( diverging lens ).
The concave lens used for correcting myopia should be of such a focal length ( or power ) that it produces a virtual image of the distant object at the far point of the myopic eye.
The whole purpose of using a concave lens here is to reduce the converging power of the eye-lens.
Calculation of power of concave lens to correct myopia :
Let distance of point from eye be x. then for lens to be used , u = -∞ , v = -x , f = ?
From lens formula :
(1/v) - 1/u= 1/f
(1/-x) - (1/-∞) = 1/f
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 ).
2. Hypermetropia ( Long-sightedness or far sightedness ) :
Symptoms :
hypermetropia is that defect of vision due to which a person cannot see the nearby objects clearly.
The near point of a hypermetropic eye is more than 25 cm away.
A person has to hold the reading material at an arm's length , much beyond 25 cm from the eye for comfortable reading.
Reason :
The cilliary muscles attached to the eye-lens become weak and cannot make the eye-lens thicker to increase its converging power. So , due to the low converging power of eye lens in an eye suffering from hypermetropia , the image of nearby object is formed behind the retina and hence the eye cannot see it clearly.
Causes :
The lens may be thinner ( low converging power of eye-lens of its large focal length.
Due to eye-ball being too short.
Correction:
Hypermetropia is corrected by using spectacles containing convex lens.
Calculation of the power of convex lens to correct Hypermetropia.
Let distance of near point from the eye be z. For lens to be used
u = -D , v = -z , f= ?
From the lens formula
1/v - 1/u = 1/f
f = zD/z-D
Where D = .25 m
This is required expression for the focal length of the convex lens to be used.
3. Presbyopia :
Presbyopia is that defect of vision due to which an old person cannot see the objects clearly due to loss of power of Accommodation of the eyes.
Symptoms: Blurred vision at normal reading distance eyestrain or headaches after reading or doing close up work.
Reason : It is caused by a Hardening of the lens of your eyes. Which occurs with aging.
Causes: it arises due to the gradual weakening of the cilliary muscles and diminishing flexibility of the eye lens sometimes , a person may suffer from both myopia and hypermetropia.
Correction : 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 ( to correct myopia ) used for distant vision and the lower part consists of a convex lens ( to correct hypermetropia ) used for reading purpose.
4. Astigmatism :
Astigmatism is a condition in which the eyes are not completely round . A person suffering from this defect cannot simultaneously focus on both horizontal and vertical lines of a wire gauze.
Reason : This defect arises due to the fact that the cornea is not perfectly Spherical.
Correction: this can be corrected by using a cylindrical lens.