Basic Facts The Human Eye and the Colourful World
Eye forms an image of an object at the retina.
For a normal eye the near point is 25cm and
far point is at infinity
Following defects are commonly observed in eyes
(i) Myopia: Myopia is also known as short sightedness or
near sightedness.Aperson suffering from myopia can
see the nearby objects clearly but distant objects are not very clear or distinct. One of the main reason for
this defect is the increase in the curvature of eye lens.
Therefore, the image of a far obejct is formed infront
of the retina while the image of nearby objects is
formed at retina.
To correct the defects of short sightedness a
concave lens of apporpriate power is placed before
the eye.
(ii) Hypermetropia: Hypermetropia is also known as long
sightednessorfar sightedness.Aperson suffering from
hypermetropia is able to see distant objects clearly.
Such persons connot see the nearby objects clearly.
o correct the defect of long sigthedness a
convex lens of appropriate power is placed before
the eye.
(iii) Presbyopia: The flexibility of eye lens and its surrounding
fibrous structure decreases with age and thereby the
accomdation power of eye reduces. This makes it
difficult for people to clearly see the nearby objects.
(iv) Astigmatism: Astigmatism is beacuse of irregular curvature
of the cornea and people of all ages can be affected by
this problem.Aperson with astigmatism cannot clearly
see horizontal and vertical lines at the same distance
simultaneouly. Cylindrical lenses are used to correct
this defect of eye.
(v) Cataraet With aging thetransperancy andflexibility of eye
lens reduces. The eye stalls reflecting light because of
this reduction in transperancy and the object is not
clearly visible to such an eye. This defect is known as
cataract