Eye

Eye

Defects Of Vision

       A person with normal sight can see clearly the objects lying between near point and the far part (infinity for a normal eye). The distance between the near point and the for point is called range of vision.

       When the object lies at the near point, the normal eye produces its image on the retina [Fig. 4.02 (a)]. The eye achieves it by accommodating the eyelens to mini- mum focal length. The eyelens is thickest at that time.


        When the object lies at the far point i.e. at infinity, the normal eye still produces its images on the retina by accommodating the eyelens to maximum focal length [Fig 4.02 (b)]. The eye lens is thinnest at that time.
        In practice, a person may suffer from the following common eye defects: 
1. Nearsightedness or Myopia: A person suffering from myopia can see only near objects clearly but cannot see the objects beyond a certain distance clearly.

    It implies that the far point is not infinity but some nearer point. In case of a myopic eye, when an object lies at infinity (at the far point for a normal eye), its image is formed in front of the retina as shown in Fig. 4.03. As a result, the image on retina is blurred or out of focus. This defect in eye occurs, when either:

(i) the eyeball is longer than its normal size i.e. when the distance between the eye lens and the retina is more than the normal distance or

(ii) the focal length of the eye lens is smaller than its normal focal length.

Remedy of the defect: The far point for a myopic eye is much nearer than infinity. If’ F’ is far point for a myopic eye, then the image of an object placed at the point ‘F' will be formed on the retina as shown in Fig.

The myopic eye will get cured against this defect, if it is able to see the objects at infinity clearly. In order to correct the eye for this defect, a concave lens of suitable focal length is placed close to the eye, so that the parallel ray of light from an object at infinity after refraction through the lens 
appear to come from the far point F' of the myopic eye as shown Fig (b).
       If x is the distance of the far point from the eye, then for the concave lens placed before the eye 
           \( u = \infty \)
    and \( v = - x \)  (\( \because \)distance is measured against the incident rays)
    Let f be the focal length of the required concave lens. From the lens formula, 
we have 
\( - \frac{1} {u} + \frac{1} {v} = \frac{1} {f} \) or \( - \frac{1} {\infty } + \frac{1} {{ - x}} = \frac{1} {f} \) or \( 0 - \frac{1} {x} = \frac{1} {f} \) or f = -x
    Thus, myopic eye is cured against the defect by using a concave lens of focal length equal to the distance of its far point from the eye.

2. Farsightedness or Hypermetropia: A person suffering from hypermetropia can see distant objects clearly, but not the nearby objects.
It implies that the near point is not at the normal distance but at some point farther from the eye. In case of a hypermetropic eye, when the object lies at the point N (at the near point for a normal eye), its image is formed behind the retina as shown in Fig. . This results into the formation of a blurred image on retina. This defect occurs, when either

(i) the eyeball is shorter than its normal size i.e. when the distance between the eye lens and the retina is less than the normal distance or

(ii) the focal length of the eye lens is larger than its normal focal length.
Remedy of the defact. The near point N' for a hypermetropic eye is farther than N, the near point for a normal eye. A hypermetropic eye forms the image of an object placed at the point N' on the retina as shown in Fig. (a).