the human eye and the colorful world

1. THE HUMAN
EYE
AND THE
COLOURFUL
WORLDPrepared by: TUSHAR
GAUR
Class: 10th B

2. Introduction
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. 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

3. Components Of Human
Eye
 The eye is made up of three coats,
enclosing three transparent structures.
The outermost layer, known as
the fibrous tunic, is composed of
the cornea and sclera. The middle layer,
known as the vascular tunic or uvea,
consists of the choroid, ciliary body,
and iris. The innermost is the retina,
which gets its circulation from the vessels
of the choroid as well as the retinal
vessels, which can be seen in an
ophthalmoscope.
 Within these coats are the aqueous
humour, the vitreous body, and the
flexible lens. The aqueous humour is a
clear fluid that is contained in two areas:
the anterior chamber between the cornea
and the iris, and the posterior
chamber between the iris and the lens.

4. Working of Eye
The individual components of the eye
work in a manner similar to a camera.
Each part plays a vital role in providing
clear vision. So think of the eye as a
camera with the cornea, behaving much
like a lens cover. As the eye 's main
focusing element, the cornea takes
widely diverging rays of light and bends
them through the pupil, the dark, round
opening in the center of the colored iris.
The iris and pupil act like the aperture of
a camera.
Next in line is the lens which acts like the
lens in a camera, helping to focus light to
the back of the eye. Note that the lens is
the part which becomes cloudy and is
removed during cataract surgery to be
replaced by an artificial implant
nowadays.

5. Function of Iris and Pupil
 The iris control the amount of light entering the
eyes. The iris automatically adjust the size of the
pupil according to the intensity of light received
by the eye. If the amount of light received by the
eye is large (as during the day time ),then the iris
contracts the pupil (makes the pupil small) and
reduces the amount of light entering the eye. On
the other hand , if the amount of light received by
the eye is small(as the room is dark)the iris
expands the pupil (make the pupil large ) so that
more light may enter the eyes.
 The adjustment of the size of pupil takes some
time .

7. Accommodation Of Power
The ability of the eye lens to see both near and distant
objects by adjusting its focal length is called the power of
accommodation of the eye.
The eye lens is composed of a fibrous jelly like
material. Its curvature can be changed to some extent by
the ciliary muscles. The change in the curvature of the
eye lens can change its focal length. When the muscles
are relaxed, the lens becomes thin and its focal length
increases and when the muscles contract, the lens
becomes thick and its focal length decreases.
Near point :-
The minimum distance
at which the eye can see
objects clearly is called
the near point or least
distance of distinct vision.
For a normal eye it is
25cm.
Far point :-
The farthest distance up
to which the eye can see
objects clearly is called
the far point of the eye.
For a normal eye it is
between 25cm and
infinity.

8. Defects Of Vision And Their
Corrections
Myopia is a defect of vision in which a person can see
nearby
objects clearly but cannot see distant objects clearly
because the
image is formed in front of the retina.
This may be due to:-
i) Increase in curvature of the eye lens
ii) Increase in the length of the eye ball

9. Hypermetropia is a defect of vision in which a person can
see
distant objects clearly but cannot see nearby objects clearly
because
the image is formed behind the retina.
This may be due to:-
i) Decrease in curvature of eye lens
ii) Decrease in the length of the eye ball

10. Presbyopia is a defect of vision in old people in which
they are not able to see nearby objects clearly due to the
increase in the distance of near point. This is due to the
weakening of the ciliary muscles and decrease in the
flexibility of the eye lens. It can be corrected by using suitable
convex lens. Sometimes they are not able to see both nearby
and distant objects clearly. It can be corrected by using
bifocal lenses consisting of both concave and convex lenses.
The upper part is concave for correction of distant vision and
the lower part is convex for correction of near vision.

11. Refraction of Light Through
Glass Prism
When a ray of light
passes through a
glass prism, it gets
bent twice at the air-
glass interface and
glass- air interface.
The emergent ray
is deviated by an
angle to the incident
ray. This angle is
called the angle of
deviation.

12. Dispersion of White Light by
glass Prism
When a beam of white light is passed through a glass
prism, it is split up into a band of colors called spectrum.
This is called dispersion of white light. The spectrum of
white has the colors violet, indigo, blue, green, yellow,
orange and red (VIBGYOR). The red light bends the least
and the violet light bends the most.
Beam of white light
Spectrum
Glass prism
R
O
Y
G
I
B
V

13. Recombination of the
Spectrum of White Light
When a beam of white light is passed through a
glass prism, it is split up into its component
colors. When these colors are allowed to fall on
an inverted glass prism it recombines to produce
white light.

14. Rainbow
A Rainbow is an optical and meteorological
phenomenon that is caused by both reflection and
refraction of light in water droplets resulting in a
spectrum of light appearing in the sky. It takes
the form of a multicolored arc . Rainbows caused
by
sunlight always appearing
the section of sky directly
opposite the sun.

15. Formation of Rainbow
1. Light from sun strikes
raindrop.
2. Some of the light is
reflected.
3. The rest of the light is
refracted.
4. White light splits into
component colors.
5. Light is refracted again
as it leaves raindrop.
6. Colors are further

16. Position And Twinkling Of
Stars
The twinkling of stars is due to
atmospheric refraction of
starlight. Since light bends
towards the normal the apparent
position of the star is slightly
different from its actual position
as it passes through the
atmosphere. Hence the star
appears slightly higher than its
actual position. Due to changing
condition of earth's atmosphere
the apparent position of the star
changes slightly and the
intensity of light reaching the
eye also fluctuates. This gives
rise to the twinkling effect of the

17. Advance Sunrise and Delayed
Sunset
The Sun is visible to us about 2 minutes before
sunrise and about two minutes after sunset due to
atmospheric refraction. The apparent flattening of the
sun’s disc at sunrise and at sunset is also due to
atmospheric refraction.

18. Scattering of Light: Tyndall
effect
When a beam of light passes
through a colloidal solution, the path
of light becomes visible due to the
scattering of light by the colloid
particles. This is known as Tyndall
effect.
The earth’s atmosphere contains
air molecules, water droplets, dust,
smoke etc. When sunlight passes
through the atmosphere the path of
the light becomes visible due to the
scattering of light by these particles.
The color of the scattered light
depends upon the size of the
scattering particles. Very fine
particles scatter blue light. Larger
particles scatter different colors of
light.

19. Why the Sky Appears Blue
?
The fine particles in the atmosphere have size smaller
than the wave length of visible light. They can scatter
blue light which has a shorter wave length than red light
which has a longer wave length. When sunlight passes
through the atmosphere, the fine particles in the
atmosphere scatter the blue color morestrongly than the red and
so the sky appears blue.
If the earth had no
atmosphere there would
not be any scattering of
light and the sky would
appear dark. The sky
appears dark at very high

21. Why Clouds Are White ?
When light beams interact with particles suspended in air,
some of the energy is scattered, which means the light
beam changes direction, and usually color as well. The
amount of light scattered is a function of the size of the
particle relative to the wavelength of light falling on it.
Cloud particles are largeenough to scatter any color of
light that falls on them. The
repe- ated scattering of light,
called multiple scattering,
causes whitish light because
enough light of all colors is
scattered to your eye, and
those colors combine to make

22. At sunrise or sunset, however, when the Sun is low on the
horizon, the light rays must pass through more of the
atmosphere – and therefore bounce off more molecules –
than at other times of day. This means that more blue light
gets scattered away before the light reaches your eyes.
Other colors – such as red,
orange and yellow – can
therefore continue to pass
through the atmosphere
unaffected, creating beautiful
colors at the start and end of
the day.
Why Sky Appears red at Sunrise
And Sunset ?

24. What We Have Learnt
 The ability of the eye to focus both near and distant
objects ,by adjusting its focal length, is called the
accommodation of the eye.
 The smallest distance, at which the eye can see
objects without strain , is called without strain is
called the near point of the eye or the least distance
of distinct vision . For a young adult with normal
vision, it is about 25 cm .
 The common refractive defects of vision include
myopia, Hypermetropia and Presbyopia.
 The splitting of white light into its component colors
is called dispersion.
 Scattering of light causes the blue color of sky and
the reddening of the sun at the sunrise and sunset.