NCERT Textbook - The Human Eye and Colourful World Class 10 Notes | EduRev

General Science(Prelims) by IRS Divey Sethi

Class 10 : NCERT Textbook - The Human Eye and Colourful World Class 10 Notes | EduRev

 Page 1


The Human Eye and
the Colourful World
11 CHAPTER
Y
ou have studied in the previous chapter about refraction of light by
lenses. You also studied the nature, position and relative size of
images formed by lenses. How can these ideas help us in the study of
the human eye? The human eye uses light and enables us to see objects
around us. It has a lens in its structure.  What is the function of the lens
in a human eye? How do the lenses used in spectacles correct defects of
vision? Let us consider these questions in this chapter.
We have learnt in the previous chapter about light and some of its
properties. In this chapter, we shall use these ideas to study some of the
optical phenomena in nature. We shall also discuss about rainbow
formation, splitting of white light and blue colour of the sky.
11.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us. On
closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch. It is, however, impossible to identify
colours while closing the eyes. Thus, of all the sense organs, the human
eye is the most significant one as it enables us to see the beautiful,
colourful world around us.
The human eye is like a camera.  Its lens
system forms an image on a light-sensitive
screen called the retina. Light enters the eye
through a thin membrane called the cornea.
It forms the transparent bulge on the front
surface of the eyeball as shown in Fig. 11.1.
The eyeball is approximately spherical in shape
with a diameter of about 2.3 cm. Most of the
refraction for the light rays entering the eye
occurs at the outer surface of the cornea. The
crystalline lens merely provides the finer
adjustment of focal length required to focus
objects at different distances on the retina. We find a structure called iris
behind the cornea. Iris is a dark muscular diaphragm that controls the
size of the pupil.  The pupil regulates and controls the amount of light
Figure 11.1
The human eye
2020-21
Page 2


The Human Eye and
the Colourful World
11 CHAPTER
Y
ou have studied in the previous chapter about refraction of light by
lenses. You also studied the nature, position and relative size of
images formed by lenses. How can these ideas help us in the study of
the human eye? The human eye uses light and enables us to see objects
around us. It has a lens in its structure.  What is the function of the lens
in a human eye? How do the lenses used in spectacles correct defects of
vision? Let us consider these questions in this chapter.
We have learnt in the previous chapter about light and some of its
properties. In this chapter, we shall use these ideas to study some of the
optical phenomena in nature. We shall also discuss about rainbow
formation, splitting of white light and blue colour of the sky.
11.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us. On
closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch. It is, however, impossible to identify
colours while closing the eyes. Thus, of all the sense organs, the human
eye is the most significant one as it enables us to see the beautiful,
colourful world around us.
The human eye is like a camera.  Its lens
system forms an image on a light-sensitive
screen called the retina. Light enters the eye
through a thin membrane called the cornea.
It forms the transparent bulge on the front
surface of the eyeball as shown in Fig. 11.1.
The eyeball is approximately spherical in shape
with a diameter of about 2.3 cm. Most of the
refraction for the light rays entering the eye
occurs at the outer surface of the cornea. The
crystalline lens merely provides the finer
adjustment of focal length required to focus
objects at different distances on the retina. We find a structure called iris
behind the cornea. Iris is a dark muscular diaphragm that controls the
size of the pupil.  The pupil regulates and controls the amount of light
Figure 11.1
The human eye
2020-21
Science
188
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.
Do You Know?
Damage to or malfunction of any part of the visual system can lead to significant loss
of visual functioning. For example, if any of the structures involved in the transmission
of light, like the cornea, pupil, eye lens, aqueous humour and vitreous humour or
those responsible for conversion of light to electrical impulse, like the retina or even
the optic nerve that transmits these impulses to the brain, is damaged, it will result
in visual impairment.You might have experienced that you are not able to see objects
clearly for some time when you enter from bright light to a room with dim light. After
sometime, however, you may be able to see things in the dim-lit room. The pupil of
an eye acts like a variable aperture whose size can be varied with the help of the iris.
When the light is very bright, the iris contracts the pupil to allow less light to enter
the eye. However, in dim light the iris expands the pupil to allow more light to enter
the eye. Thus, the pupil opens completely through the relaxation of the iris.
11.1.1 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.
2020-21
Page 3


The Human Eye and
the Colourful World
11 CHAPTER
Y
ou have studied in the previous chapter about refraction of light by
lenses. You also studied the nature, position and relative size of
images formed by lenses. How can these ideas help us in the study of
the human eye? The human eye uses light and enables us to see objects
around us. It has a lens in its structure.  What is the function of the lens
in a human eye? How do the lenses used in spectacles correct defects of
vision? Let us consider these questions in this chapter.
We have learnt in the previous chapter about light and some of its
properties. In this chapter, we shall use these ideas to study some of the
optical phenomena in nature. We shall also discuss about rainbow
formation, splitting of white light and blue colour of the sky.
11.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us. On
closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch. It is, however, impossible to identify
colours while closing the eyes. Thus, of all the sense organs, the human
eye is the most significant one as it enables us to see the beautiful,
colourful world around us.
The human eye is like a camera.  Its lens
system forms an image on a light-sensitive
screen called the retina. Light enters the eye
through a thin membrane called the cornea.
It forms the transparent bulge on the front
surface of the eyeball as shown in Fig. 11.1.
The eyeball is approximately spherical in shape
with a diameter of about 2.3 cm. Most of the
refraction for the light rays entering the eye
occurs at the outer surface of the cornea. The
crystalline lens merely provides the finer
adjustment of focal length required to focus
objects at different distances on the retina. We find a structure called iris
behind the cornea. Iris is a dark muscular diaphragm that controls the
size of the pupil.  The pupil regulates and controls the amount of light
Figure 11.1
The human eye
2020-21
Science
188
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.
Do You Know?
Damage to or malfunction of any part of the visual system can lead to significant loss
of visual functioning. For example, if any of the structures involved in the transmission
of light, like the cornea, pupil, eye lens, aqueous humour and vitreous humour or
those responsible for conversion of light to electrical impulse, like the retina or even
the optic nerve that transmits these impulses to the brain, is damaged, it will result
in visual impairment.You might have experienced that you are not able to see objects
clearly for some time when you enter from bright light to a room with dim light. After
sometime, however, you may be able to see things in the dim-lit room. The pupil of
an eye acts like a variable aperture whose size can be varied with the help of the iris.
When the light is very bright, the iris contracts the pupil to allow less light to enter
the eye. However, in dim light the iris expands the pupil to allow more light to enter
the eye. Thus, the pupil opens completely through the relaxation of the iris.
11.1.1 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.
2020-21
The Human Eye and the Colourful World 189
Why do we have two eyes for vision and not just one?
There are several advantages of our having two eyes instead of one. It gives a wider
field of view. A human being has a horizontal field of view of about 150° with one eye
and of about 180° with two eyes. The ability to detect faint objects is, of course,
enhanced with two detectors instead of one.
Some animals, usually prey animals, have their two eyes positioned on opposite sides
of their heads to give the widest possible field of view. But our two eyes are positioned
on the front of our heads, and it thus reduces our field of view in favour of what is
called stereopsis. Shut one eye and the world looks flat – two-dimensional. Keep both
eyes open and the world takes on the third dimension of depth. Because our eyes
are separated by a few centimetres, each eye sees a slightly different image. Our
brain combines the two images into one, using the extra information to tell us how
close or far away things are.
11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION
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.
(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. 11.2 (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 is illustrated in Fig. 11.2 (c).
A concave lens of suitable power
will bring the image back on to
the retina and thus the defect
is corrected.
Do You Know?
Figure 11.2 Figure 11.2 Figure 11.2 Figure 11.2 Figure 11.2
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens
2020-21
Page 4


The Human Eye and
the Colourful World
11 CHAPTER
Y
ou have studied in the previous chapter about refraction of light by
lenses. You also studied the nature, position and relative size of
images formed by lenses. How can these ideas help us in the study of
the human eye? The human eye uses light and enables us to see objects
around us. It has a lens in its structure.  What is the function of the lens
in a human eye? How do the lenses used in spectacles correct defects of
vision? Let us consider these questions in this chapter.
We have learnt in the previous chapter about light and some of its
properties. In this chapter, we shall use these ideas to study some of the
optical phenomena in nature. We shall also discuss about rainbow
formation, splitting of white light and blue colour of the sky.
11.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us. On
closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch. It is, however, impossible to identify
colours while closing the eyes. Thus, of all the sense organs, the human
eye is the most significant one as it enables us to see the beautiful,
colourful world around us.
The human eye is like a camera.  Its lens
system forms an image on a light-sensitive
screen called the retina. Light enters the eye
through a thin membrane called the cornea.
It forms the transparent bulge on the front
surface of the eyeball as shown in Fig. 11.1.
The eyeball is approximately spherical in shape
with a diameter of about 2.3 cm. Most of the
refraction for the light rays entering the eye
occurs at the outer surface of the cornea. The
crystalline lens merely provides the finer
adjustment of focal length required to focus
objects at different distances on the retina. We find a structure called iris
behind the cornea. Iris is a dark muscular diaphragm that controls the
size of the pupil.  The pupil regulates and controls the amount of light
Figure 11.1
The human eye
2020-21
Science
188
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.
Do You Know?
Damage to or malfunction of any part of the visual system can lead to significant loss
of visual functioning. For example, if any of the structures involved in the transmission
of light, like the cornea, pupil, eye lens, aqueous humour and vitreous humour or
those responsible for conversion of light to electrical impulse, like the retina or even
the optic nerve that transmits these impulses to the brain, is damaged, it will result
in visual impairment.You might have experienced that you are not able to see objects
clearly for some time when you enter from bright light to a room with dim light. After
sometime, however, you may be able to see things in the dim-lit room. The pupil of
an eye acts like a variable aperture whose size can be varied with the help of the iris.
When the light is very bright, the iris contracts the pupil to allow less light to enter
the eye. However, in dim light the iris expands the pupil to allow more light to enter
the eye. Thus, the pupil opens completely through the relaxation of the iris.
11.1.1 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.
2020-21
The Human Eye and the Colourful World 189
Why do we have two eyes for vision and not just one?
There are several advantages of our having two eyes instead of one. It gives a wider
field of view. A human being has a horizontal field of view of about 150° with one eye
and of about 180° with two eyes. The ability to detect faint objects is, of course,
enhanced with two detectors instead of one.
Some animals, usually prey animals, have their two eyes positioned on opposite sides
of their heads to give the widest possible field of view. But our two eyes are positioned
on the front of our heads, and it thus reduces our field of view in favour of what is
called stereopsis. Shut one eye and the world looks flat – two-dimensional. Keep both
eyes open and the world takes on the third dimension of depth. Because our eyes
are separated by a few centimetres, each eye sees a slightly different image. Our
brain combines the two images into one, using the extra information to tell us how
close or far away things are.
11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION
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.
(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. 11.2 (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 is illustrated in Fig. 11.2 (c).
A concave lens of suitable power
will bring the image back on to
the retina and thus the defect
is corrected.
Do You Know?
Figure 11.2 Figure 11.2 Figure 11.2 Figure 11.2 Figure 11.2
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens
2020-21
Science
190
(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. 11.3 (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. 11.3 (c). Eye-glasses
with converging lenses provide the additional focussing
power required for forming the image on the retina.
(c)  Presbyopia
The power of accommodation of the eye usually
decreases with ageing. For most people, the near point
gradually recedes away. They find it difficult to see
nearby objects comfortably and distinctly without
corrective eye-glasses.  This defect is called Presbyopia.
It arises due to the gradual weakening of the ciliary
muscles and diminishing flexibility of the eye lens.
Sometimes, a person may suffer from both myopia and
hypermetropia.  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. It facilitates distant
vision. The lower part is a convex lens.  It facilitates near vision.
  These days, it is possible to correct the refractive defects with
contact lenses or through surgical interventions.
Figure 11.3 Figure 11.3 Figure 11.3 Figure 11.3 Figure 11.3
(a), (b) The hypermetropic eye, and (c)
correction for hypermetropia
N = Near point of a
      hypermetropic eye.
N’ = Near point of a
      normal eye.
QUESTIONS
?
1. What is meant by power of accommodation of the eye?
2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly.
What should be the type of the corrective lens used to restore proper
vision?
3. What is the far point and near point of the human eye with normal
vision?
4. A student has difficulty reading the blackboard while sitting in the last
row. What could be the defect the child is suffering from? How can it be
corrected?
2020-21
Page 5


The Human Eye and
the Colourful World
11 CHAPTER
Y
ou have studied in the previous chapter about refraction of light by
lenses. You also studied the nature, position and relative size of
images formed by lenses. How can these ideas help us in the study of
the human eye? The human eye uses light and enables us to see objects
around us. It has a lens in its structure.  What is the function of the lens
in a human eye? How do the lenses used in spectacles correct defects of
vision? Let us consider these questions in this chapter.
We have learnt in the previous chapter about light and some of its
properties. In this chapter, we shall use these ideas to study some of the
optical phenomena in nature. We shall also discuss about rainbow
formation, splitting of white light and blue colour of the sky.
11.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us. On
closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch. It is, however, impossible to identify
colours while closing the eyes. Thus, of all the sense organs, the human
eye is the most significant one as it enables us to see the beautiful,
colourful world around us.
The human eye is like a camera.  Its lens
system forms an image on a light-sensitive
screen called the retina. Light enters the eye
through a thin membrane called the cornea.
It forms the transparent bulge on the front
surface of the eyeball as shown in Fig. 11.1.
The eyeball is approximately spherical in shape
with a diameter of about 2.3 cm. Most of the
refraction for the light rays entering the eye
occurs at the outer surface of the cornea. The
crystalline lens merely provides the finer
adjustment of focal length required to focus
objects at different distances on the retina. We find a structure called iris
behind the cornea. Iris is a dark muscular diaphragm that controls the
size of the pupil.  The pupil regulates and controls the amount of light
Figure 11.1
The human eye
2020-21
Science
188
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.
Do You Know?
Damage to or malfunction of any part of the visual system can lead to significant loss
of visual functioning. For example, if any of the structures involved in the transmission
of light, like the cornea, pupil, eye lens, aqueous humour and vitreous humour or
those responsible for conversion of light to electrical impulse, like the retina or even
the optic nerve that transmits these impulses to the brain, is damaged, it will result
in visual impairment.You might have experienced that you are not able to see objects
clearly for some time when you enter from bright light to a room with dim light. After
sometime, however, you may be able to see things in the dim-lit room. The pupil of
an eye acts like a variable aperture whose size can be varied with the help of the iris.
When the light is very bright, the iris contracts the pupil to allow less light to enter
the eye. However, in dim light the iris expands the pupil to allow more light to enter
the eye. Thus, the pupil opens completely through the relaxation of the iris.
11.1.1 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.
2020-21
The Human Eye and the Colourful World 189
Why do we have two eyes for vision and not just one?
There are several advantages of our having two eyes instead of one. It gives a wider
field of view. A human being has a horizontal field of view of about 150° with one eye
and of about 180° with two eyes. The ability to detect faint objects is, of course,
enhanced with two detectors instead of one.
Some animals, usually prey animals, have their two eyes positioned on opposite sides
of their heads to give the widest possible field of view. But our two eyes are positioned
on the front of our heads, and it thus reduces our field of view in favour of what is
called stereopsis. Shut one eye and the world looks flat – two-dimensional. Keep both
eyes open and the world takes on the third dimension of depth. Because our eyes
are separated by a few centimetres, each eye sees a slightly different image. Our
brain combines the two images into one, using the extra information to tell us how
close or far away things are.
11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION 11.2 DEFECTS OF VISION AND THEIR CORRECTION
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.
(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. 11.2 (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 is illustrated in Fig. 11.2 (c).
A concave lens of suitable power
will bring the image back on to
the retina and thus the defect
is corrected.
Do You Know?
Figure 11.2 Figure 11.2 Figure 11.2 Figure 11.2 Figure 11.2
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens
2020-21
Science
190
(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. 11.3 (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. 11.3 (c). Eye-glasses
with converging lenses provide the additional focussing
power required for forming the image on the retina.
(c)  Presbyopia
The power of accommodation of the eye usually
decreases with ageing. For most people, the near point
gradually recedes away. They find it difficult to see
nearby objects comfortably and distinctly without
corrective eye-glasses.  This defect is called Presbyopia.
It arises due to the gradual weakening of the ciliary
muscles and diminishing flexibility of the eye lens.
Sometimes, a person may suffer from both myopia and
hypermetropia.  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. It facilitates distant
vision. The lower part is a convex lens.  It facilitates near vision.
  These days, it is possible to correct the refractive defects with
contact lenses or through surgical interventions.
Figure 11.3 Figure 11.3 Figure 11.3 Figure 11.3 Figure 11.3
(a), (b) The hypermetropic eye, and (c)
correction for hypermetropia
N = Near point of a
      hypermetropic eye.
N’ = Near point of a
      normal eye.
QUESTIONS
?
1. What is meant by power of accommodation of the eye?
2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly.
What should be the type of the corrective lens used to restore proper
vision?
3. What is the far point and near point of the human eye with normal
vision?
4. A student has difficulty reading the blackboard while sitting in the last
row. What could be the defect the child is suffering from? How can it be
corrected?
2020-21
The Human Eye and the Colourful World 191
Think it over
You talk of wondrous things you see,
You say the sun shines bright;
I feel him warm, but how can he
Or make it day or night?
 – C. CIBBER
Do you know that our eyes can live even after our death? By donating our eyes after we
die, we can light the life of a blind person.
About 35 million people in the developing world are blind and most of them can be
cured. About 4.5 million people with corneal blindness can be cured through corneal
transplantation of donated eyes. Out of these 4.5 million, 60% are children below the
age of 12. So, if we have got the gift of vision, why not pass it on to somebody who does
not have it? What do we have to keep in mind when eyes have to be donated?
n Eye donors can belong to any age group or sex. People who use spectacles, or those
operated for cataract, can still donate the eyes. People who are diabetic, have
hypertension, asthma patients and those without communicable diseases can also
donate eyes.
n Eyes must be removed within 4-6 hours after death. Inform the nearest eye bank
immediately.
n The eye bank team will remove the eyes at the home of the deceased or at a hospital.
n Eye removal takes only 10-15 minutes. It is a simple process and does not lead to
any disfigurement.
n Persons who were infected with or died because of AIDS, Hepatitis B or C, rabies,
acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate eyes.
An eye bank collects, evaluates and distributes the donated eyes. All eyes donated are
evaluated using strict medical standards. Those donated eyes found unsuitable for
transplantation are used for valuable research and medical education. The identities of
both the donor and the recipient remain confidential.
One pair of eyes gives vision to up to FOUR CORNEAL BLIND PEOPLE.
11.3 REFRACTION OF LIGHT THROUGH A PRISM 11.3 REFRACTION OF LIGHT THROUGH A PRISM 11.3 REFRACTION OF LIGHT THROUGH A PRISM 11.3 REFRACTION OF LIGHT THROUGH A PRISM 11.3 REFRACTION OF LIGHT THROUGH A PRISM
You have learnt how light gets refracted through a rectangular glass
slab. For parallel refracting surfaces, as in a glass slab, the emergent ray
is parallel to the incident ray.  However, it is slightly displaced laterally.
How would light get refracted through a transparent prism? Consider a
triangular glass prism. It has two triangular bases and three rectangular
lateral surfaces. These surfaces are inclined to each other. The angle
between its two lateral faces is called the angle of the prism. Let us now
do an activity to study the refraction of light through a triangular glass
prism.
2020-21
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