The human eye functions similarly to a camera, with light entering through the cornea and being focused onto the retina. The lens allows for accommodation to view objects at different distances. The retina contains light-sensitive cells that send signals to the brain for visual perception. Common vision defects like myopia and hyperopia can be corrected using concave and convex lenses, respectively. When light passes through water droplets in the atmosphere, it is dispersed into colors via refraction and reflection, causing phenomena like rainbows and the blue color of the sky. Atmospheric scattering and refraction also contribute to the twinkling of stars and the delayed sunset.

1. The Human Eye And The
Colourful World
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3. ● The human eye is like a camera.
● Its lens forms an image on a light-sensitive screen called the retina.
● Light enters the eye through a thin membrane called the cornea.
● Cornea 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 of 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.
● Iris is a dark muscular diaphragm behind the cornea and it controls the size of the pupil.
● The pupil regulates and controls the amount of light entering the eye.
● The eye lens forms an inverted real image of the object on the retina.
Parts and functions of a Human Eye
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4. ● 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 curvature can thus change the focal length of the lens.
● The retina is a delicate membrane having enormous number of light- sensitive cells.
● The light-sensitive cells get activated upon illumination and generate electric signals.
● These signals are sent to brain via optic nerves.
● The brain intercepts these signals and finally processes the information for our
perception.
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5. Power of Accommodation
 The ability of the eye lens to adjust its focal length is called accommodation.
 The eye lens is composed of a fibrous, jelly-like material and its curvature can be modified
by the ciliary muscles. Hence, the focal length can be changed as per the requirement.
 When the muscles are relaxed, the lens becomes thin. The radius of curvature and hence
the focal length increases. This enables us to see the distant objects clearly.
 When we look at the objects closer to they, ciliary muscles contract decreasing the radius
of curvature and hence the focal length. This enables us to see the nearby objects clearly.
The minimum distance, at which objects can be seen most distinctly
without strain, is called Least Distance of Distinct Vision (LDDV). For
a normal eye, LDDV is 25 cm.
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6. DEFECTS OF VISION AND THEIR
CORRECTION
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7. Myopia (Short-sightedness)
● Myopia is also known as near-sightedness.
● A person with myopic eye can see nearby objects clearly but cannot see distant objects
distinctly.
● Such a person may clearly see upto a distance of a few metres.
● In myopic eye, the image of a distant object is formed in front of the retina and not on the
retinal itself.
This defect may arise due to
(i) excessive curvature of the eye lens (short focal length of the eye lens)
(OR)
(ii) Elongation of the eyeball.
Myopia can be corrected by using a concave lens of suitable power(focal length).
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9. LDDV = 25 cm
O
LDDV = 25 cm
I
O
LDDV = 25 cm
Myopic Eye
O
I
I
O
LDDV = 25 cm
I
O
LDDV = 25 cm
II
Near Point
Myopic Eye is corrected with Concave Lens
Normal Eye
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10. Hypermetropia (Long-sightedness)
● Hypermetropia is also known as far-sightedness.
● A person with hypermetropia can see distant objects clearly but cannot see nearby objects
distinctly.
● Such a person may has to keep a reading material much beyond 25 cm from the eye for
comfortable reading.
● In hypermetropic eye, the image of a nearby object is formed behind the retina and not on the
retinal itself.
This defect may arise due to
(i) long focal length of the eye lens or
(ii) Very small size of the eyeball.
Hypermetropia can be corrected by using a convex lens of suitable power (focal length).
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12. LDDV = 25 cm
Hypermetropic Eye
O
Far Point
LDDV = 25 cm
I
O
O
LDDV = 25 cm
I
I
LDDV = 25 cm
I
O
LDDV = 25 cm
O
II
Hypermetropic Eye is corrected with Convex Lens
Normal Eye
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13. Presbyopia
 The power of accommodation of the eye usually decreases with ageing.
 People can not see nearby objects comfortably and distinctly without corrective eye-glasses.
 This defect is called presbyopia.
It arises due to
(i) gradual weakening of the ciliary muscles and
(ii) diminishing flexibility of the eye lens.
Sometimes, a person may suffer from both myopia and hypermetropia. Such people require
bi-focal lenses which consists of both concave and convex lenses. The upper portion is
concave for distant vision and the lower portion is convex for near vision.
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15. N1
N2
Eye
A
Refracting Surfaces
i
e
REFRACTION OF LIGHT THROUGH A TRIANGULAR
PRISM
Q
P
S
R
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16. Refraction of Light through Prism:
A
Refracting Surfaces
Prism
i
δ
A
B C
e
Or1 r2
N1 N2
G
μ
Q
P S
R
<i + <e = <A + δ
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17. DISPERSION OF WHITE LIGHT THROUGH A PRISM
The phenomenon of splitting a ray of white light into its constituent colours (wavelengths) is
called dispersion and the band of colours from violet to red is called spectrum (VIBGYOR).
δr
A
B C
D
White
light
δv
Screen
N
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18. A
B C
White
light
N
A
B’ C’ White
light
Recombination of spectrum of white
light
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19. ■ A rainbow is a natural spectrum which is caused by dispersion of sunlight by tiny
water droplets present in the atmosphere after a rain shower.
■ The water droplet acts as a small prism. The incident sunlight with suitable angle
of incidence is refracted, dispersed, internally reflected and finally refracted out by
the rain drops.
■ Due to the dispersion and internal reflection, different colours reach the eye of the
observer.
■ A rainbow is always formed in a direction opposite to that of the Sun.
RAINBOW
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20. Eye
41º
43º
Sunlight
Rain drop
Formation of Rainbow
A line parallel to Sun’s ray
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21. RAINBOW
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22. ATMOSPHERIC REFRACTION
Flickering of objects above a fire:
• The apparent random wavering or flickering of objects can
be seen through a turbulent stream of hot air rising above a
fire.
• The air just above the fire becomes hotter than the further
up. The hotter air is lighter than the cooler air above it, and
has a refractive index slightly less than that of the cooler air.
• Since the physical conditions of the refracting medium (air)
are not stationary, the apparent position of the object, as
seen through the hot air, fluctuates. This wavering is
therefore due to an effect of atmospheric refraction on a
small scale in the local environment.
Refraction of light by earth’s atmosphere is called atmospheric refraction.
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23. Twinkling of Star
 The twinkling of a star is due to atmospheric refraction of starlight.
 The atmospheric refraction occurs in a medium of gradually changing
refractive index.
 Since the atmosphere bends starlight towards the normal, the apparent
position of the star is slightly different from its actual position.
 The star appears slightly higher (above) than its actual position when
viewed near the horizon.
 This apparent position is not stationary, but keeps on changing slightly, since
the physical conditions of the earth’s atmosphere are not stationary.
 Since the stars are very distant, they approximate point-sized sources of
light.
 As the path of rays of light coming from the star goes on varying slightly, the
apparent position of the star fluctuates and the amount of light entering the
eye flickers- the star sometimes appear brighter, and at some other time,
fainter which gives the twinkling effect.
Eye
Density of
Atmosphere
& Refractive
index
increase
Apparent position
of the Star
Real
position of
the Star
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24. Why Planets do not twinkle?
The planets are much closer to the earth, and are thus seen as extended sources.
Since it is the collection of large number of point-sized sources of light, the total
variation in the amount of light entering into the eye from all the individual sources
will average out to zero, thereby nullifying the twinkling effect.
Advance Sunrise and
Delayed Sunset:
The Sun is visible to us
about 2 minutes before
the actual sunrise, and
about 2 minutes after the
actual sunset because of
atmospheric refraction.
Earth
Atmosphere
Horizon
Apparent position
of the Sun
Real position
of the Sun
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25. Earth
Observer
SunriseSunset
Apparent positionApparent position
Atmosphere
Horizon Horizon
Real position Real position
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26. SCATTERING OF LIGHT
● The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles
include smoke, tiny water droplets, suspended particles of dust and molecules of air.
● When a beam of light strikes such fine particles, the path of the beam becomes visible.
● The light reaches us, after being reflected diffusedly by these particles.
● The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall
Effect.
● Tyndall Effect can be seen when a fine beam of sunlight enters a smoke-filled room
through a small hole. In this, scattering of light makes the particles visible.
● It can also be seen when sunlight passes through a canopy of a dense forest. In this, tiny
water droplets in the mist scatter light.
● The colour of the scattered light depends on the size of the scattering particles.
● Very fine particles scatter mainly blue light while particles of larger size scatter light of
longer wavelengths. If the size of the scattering particles is large enough, then, the
scattered light may even appear white.
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27. SCATTERING OF LIGHT
S
L1 ScreenL2
I
Sodium thio sulphate
solution (hypo)
Conc. Sulphuric acid
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29. SCATTERING OF LIGHT BY TINY WATER DROPLETS IN THE MIST
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30. SCATTERING OF LIGHT BY SMOKE & COLLOIDAL
PARTICLES
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31. Earth
Atmosphere
Horizon
Scattering of Light
Blue colour of the sky and Reddish appearance of the Sun at Sun-rise and Sun-set
Other colours
mostly scattered
Less Blue colour
is scattered
Blue scattered away
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32. The molecules of the atmosphere and other particles that are smaller than the longest
wavelength of visible light are more effective in scattering light of shorter wavelengths
than light of longer wavelengths. The amount of scattering is inversely proportional to
the fourth power of the wavelength. (Rayleigh Effect)
Light from the Sun near the horizon passes through thicker layers of air and a greater
distance in the Earth’s atmosphere than the light received when the Sun is overhead.
The correspondingly greater scattering of short wavelengths accounts for the reddish
appearance of the Sun at rising and at setting.
When looking at the sky in a direction away from the Sun, we receive scattered sunlight
in which short wavelengths predominate giving the sky its characteristic bluish colour.
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34. DONE BY:-
VISHNU
BHARATH
JEEVAN
ABSHEER
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