Page 168 - NCERT Science Class 10 English Medium

(a) Myopia
Myopia is also known as near-
sightedness. A person with myopia
can see nearby objects clearly but
cannot see distant objects distinctly.
A person with this defect has the far
point nearer than infinity. Such a
person may see clearly upto a
distance of a few metres. In a myopic
eye, the image of a distant object is
formed in front of the retina [Fig.
10.2 (b)] and not at the retina itself.
This defect may arise due to (i)
excessive curvature of the eye lens,
or (ii) elongation of the eyeball. This
defect can be corrected by using a
concave lens of suitable power. This
Figure
Figure 10.2
Figure
Figure 10.210.2
10.2
Figure 10.2
is illustrated in Fig. 10.2 (c). A
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens of suitable power will concave lens
bring the image back on to the
retina and thus the defect is corrected.
(b) Hypermetropia
Hypermetropia is also known as far-sightedness.
A person with hypermetropia can see distant
objects clearly but cannot see nearby objects
distinctly. The near point, for the person, is farther
away from the normal near point (25 cm). Such a
person has to keep a reading material much
beyond 25 cm from the eye for comfortable
reading. This is because the light rays from a
closeby object are focussed at a point behind the
retina as shown in Fig. 10.3 (b). This defect arises
either because (i) the focal length of the eye lens is
too long, or (ii) the eyeball has become too small.
This defect can be corrected by using a convex
lens of appropriate power. This is illustrated in
Fig. 10.3 (c). Eye-glasses with converging lenses
provide the additional focussing power required
for forming the image on the retina.

(c) Presbyopia
Figure 10.3
Figure
Figure
10.3
Figure 10.310.3
Figure 10.3
The power of accommodation of the eye usually
(a), (b) The hypermetropic eye, and (c)
decreases with ageing. For most people, the near correction for hypermetropia
point gradually recedes away. They find it difficult N = Near point of a
to see nearby objects comfortably and distinctly hypermetropic eye.
without corrective eye-glasses. This defect is N’ = Near point of a
called Presbyopia. It arises due to the gradual normal eye.

The Human Eye and the Colourful World 163

2024-25