Page 167 - NCERT Science Class 10 English Medium

entering the eye. The eye lens forms an inverted real image of the object
on the retina. The retina is a delicate membrane having enormous
number of light-sensitive cells. The light-sensitive cells get activated
upon illumination and generate electrical signals. These signals are
sent to the brain via the optic nerves. The brain interprets these signals,
and finally, processes the information so that we perceive objects as
they are.

10.1.1 Power of Accommodation
The eye lens is composed of a fibrous, jelly-like material. Its curvature
can be modified to some extent by the ciliary muscles. The change in the
curvature of the eye lens can thus change its focal length. When the
muscles are relaxed, the lens becomes thin. Thus, its focal length
increases. This enables us to see distant objects clearly. When you are
looking at objects closer to the eye, the ciliary muscles contract. This
increases the curvature of the eye lens. The eye lens then becomes thicker.
Consequently, the focal length of the eye lens decreases. This enables
us to see nearby objects clearly.
The ability of the eye lens to adjust its focal length is called
accommodation. However, the focal length of the eye lens cannot be
decreased below a certain minimum limit. Try to read a printed page by
holding it very close to your eyes. You may see the image being blurred
or feel strain in the eye. To see an object comfortably and distinctly, you
must hold it at about 25 cm from the eyes. The minimum distance, at
which objects can be seen most distinctly without strain, is called the
least distance of distinct vision. It is also called the near point of the eye.
For a young adult with normal vision, the near point is about
25 cm. The farthest point upto which the eye can see objects clearly is
called the far point of the eye. It is infinity for a normal eye. You may note
here a normal eye can see objects clearly that are between 25 cm and
infinity.
Sometimes, the crystalline lens of people at old age becomes milky and
cloudy. This condition is called cataract. This causes partial or complete
loss of vision. It is possible to restore vision through a cataract surgery.

CORRECTION
OF
VISION
AND
THEIR
DEFECTS OF VISION AND THEIR CORRECTION
10.2
10.2
10.2 DEFECTS OF VISION AND THEIR CORRECTIONDEFECTS OF VISION AND THEIR CORRECTION
10.2 DEFECTS OF VISION AND THEIR CORRECTION
10.2 DEFECTS
Sometimes, the eye may gradually lose its power of accommodation.
In such conditions, the person cannot see the objects distinctly and
comfortably. The vision becomes blurred due to the refractive defects
of the eye.
There are mainly three common refractive defects of vision. These
are (i) myopia or near-sightedness, (ii) Hypermetropia or far-
sightedness, and (iii) Presbyopia. These defects can be corrected by
the use of suitable spherical lenses. We discuss below these defects
and their correction.

162 Science

2024-25