NCERT Textbook: The Human Eye & the Colourful World | Science & Technology for UPSC CSE PDF Download

 Page 1


The Human Eye and
the Colourful World
10 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.
10.1 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. 10.1.
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 10.1 Figure 10.1 Figure 10.1 Figure 10.1 Figure 10.1
The human eye
2024-25
Page 2


The Human Eye and
the Colourful World
10 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.
10.1 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. 10.1.
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 10.1 Figure 10.1 Figure 10.1 Figure 10.1 Figure 10.1
The human eye
2024-25
Science
162
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.
10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 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.
2024-25
Page 3


The Human Eye and
the Colourful World
10 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.
10.1 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. 10.1.
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 10.1 Figure 10.1 Figure 10.1 Figure 10.1 Figure 10.1
The human eye
2024-25
Science
162
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.
10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 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.
2024-25
The Human Eye and the Colourful World 163
(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
is illustrated in Fig. 10.2 (c). A
concave lens of suitable power will
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
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
Figure 10.2 Figure 10.2 Figure 10.2 Figure 10.2 Figure 10.2
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens
Figure 10.3 Figure 10.3 Figure 10.3 Figure 10.3 Figure 10.3
(a), (b) The hypermetropic eye, and (c)
correction for hypermetropia
N = Near point of a
      hypermetropic eye.
N’ = Near point of a
      normal eye.
2024-25
Page 4


The Human Eye and
the Colourful World
10 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.
10.1 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. 10.1.
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 10.1 Figure 10.1 Figure 10.1 Figure 10.1 Figure 10.1
The human eye
2024-25
Science
162
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.
10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 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.
2024-25
The Human Eye and the Colourful World 163
(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
is illustrated in Fig. 10.2 (c). A
concave lens of suitable power will
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
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
Figure 10.2 Figure 10.2 Figure 10.2 Figure 10.2 Figure 10.2
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens
Figure 10.3 Figure 10.3 Figure 10.3 Figure 10.3 Figure 10.3
(a), (b) The hypermetropic eye, and (c)
correction for hypermetropia
N = Near point of a
      hypermetropic eye.
N’ = Near point of a
      normal eye.
2024-25
Science
164
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.
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.
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?
2024-25
Page 5


The Human Eye and
the Colourful World
10 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.
10.1 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. 10.1.
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 10.1 Figure 10.1 Figure 10.1 Figure 10.1 Figure 10.1
The human eye
2024-25
Science
162
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.
10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 DEFECTS OF VISION AND THEIR CORRECTION 10.2 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.
2024-25
The Human Eye and the Colourful World 163
(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
is illustrated in Fig. 10.2 (c). A
concave lens of suitable power will
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
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
Figure 10.2 Figure 10.2 Figure 10.2 Figure 10.2 Figure 10.2
(a), (b) The myopic eye, and (c) correction for myopia with a
concave lens
Figure 10.3 Figure 10.3 Figure 10.3 Figure 10.3 Figure 10.3
(a), (b) The hypermetropic eye, and (c)
correction for hypermetropia
N = Near point of a
      hypermetropic eye.
N’ = Near point of a
      normal eye.
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Science
164
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.
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.
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?
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The Human Eye and the Colourful World 165
10.3 REFRACTION OF LIGHT THROUGH A PRISM 10.3 REFRACTION OF LIGHT THROUGH A PRISM 10.3 REFRACTION OF LIGHT THROUGH A PRISM 10.3 REFRACTION OF LIGHT THROUGH A PRISM 10.3 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.
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.
Activity 10.1 Activity 10.1 Activity 10.1 Activity 10.1 Activity 10.1
n Fix a sheet of white paper on a drawing board using drawing pins.
n Place a glass prism on it in such a way that it rests on its triangular
base. Trace the outline of the prism using a pencil.
n Draw a straight line PE inclined to one of the refracting surfaces,
say AB, of the prism.
n Fix two pins, say at points P and Q, on the line PE as shown in
Fig. 10.4.
n Look for the images of the pins, fixed at P and Q, through the
other face AC.
n Fix two more pins, at points R and S, such that the pins at R and
S and the images of the pins at P and Q lie on the same straight
line.
n Remove the pins and the glass prism.
n The line PE meets the boundary of the prism at point E
(see Fig. 10.4). Similarly, join and produce the points R and S.  Let
these lines meet the boundary of the prism at E and F, respectively.
Join E and F.
n Draw perpendiculars to the refracting surfaces AB and AC of the
prism at points E and F, respectively.
n Mark the angle of incidence (?i), the angle of refraction (?r) and
the angle of emergence (?e) as shown in Fig. 10.4.
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