THE HUMAN EYE AND THE COLOURFUL WORLD

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THE HUMAN EYE AND THE COLOURFUL WORLD

• The Human Eye
• The Parts and their Functions of a Human Eye
• Power of Accommodation, LDDV and Far Point
• Defects of Vision and their Correction -
• Myopia, Hypermetropia and Presbyopia
• Refraction through a Prism
• Expression for Refractive Index of Prism
• Dispersion
• Rainbow
• Atmospheric Refraction Tyndall Effect, Apparent
  position Twinkling of Stars, Delayed Sunrise
  Sunset
• Scattering of Light - Blue Colour of the Sky and
  Red Colour of the Sun

Created by C. Mani, Principal, K V No.1, AFS,
Jalahalli West, Bangalore
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THE HUMAN EYE
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Cross Section through a Human Eye
Aqueous humour
Pupil
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Cross Section through a Human Eye
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Parts and functions of a Human Eye

• 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.

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• 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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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.
Least Distance of Distinct Vision (LDDV)
The minimum distance, at which objects can be
seen most distinctly without strain, is called
Least Distance of Distinct Vision. For a normal
eye, LDDV is 25 cm.
Far Point
The farthest point upto which the eye can see
objects clearly is called far point of the eye.
Far point for a normal eye is infinity. A normal
eye can see objects clearly that are between 25
cm and infinity.
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DEFECTS OF VISION AND THEIR CORRECTION
Myopia or Short-sightedness or Near-sightedness

• A person with myopic eye 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 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
• 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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Myopic Eye
Near Point
Myopic Eye corrected with Concave Lens
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Hypermetropia or Long-sightedness or
Far-sightedness
A person with hypermetropia can see distant
objects clearly but cannot see nearby objects
distinctly. A person with this defect has the
near point farther away from the normal near
point (25 cm). Such a person may has to keep a
reading material much beyond 25 cm from the eye
for comfortable reading. In myopic 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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Hypermetropic Eye
Near Point
Hypermetropic Eye corrected with Convex Lens
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Presbyopia

• The power of accommodation of the eye usually
  decreases with ageing.
• For most of the people, near point gradually
  recedes away.
• They can not see nearby objects comfortably and
  distinctly without corrective
• eye-glasses.
• This defect is called presbyopia.
• It arises due to
• 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 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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REFRACTION OF LIGHT THROUGH A TRIANGULAR PRISM -
Activity
Prism
S
R
P
Q
N2
N1
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Refraction of Light through Prism
N1
N2
d
D
e
i
O
r1
r2
µ
Prism
Refracting Surfaces
d
dm
0
i e
i
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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).
A
dr
D
N
dv
ROYGB I V
White light
B
C
Screen
Cause of Dispersion
Since µv gt µr , rr gt rv So, the
colours are refracted at different angles and
hence get separated.
and
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Dispersion can also be explained on the basis of
Cauchys equation.
(where a, b and c are constants for the material)
Since ?v lt ? r , µv gt µr But d A (µ
1) Therefore, dv gt dr So, the
colours get separated with different angles of
deviation. Violet is most deviated and Red is
least deviated.
Recombination of spectrum of white light
White light
A
White light
C
B
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RAINBOW
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Formation of Primary Rainbow
Rain drop
Sunlight
43º
41º
A line parallel to Suns ray
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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 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.There are
primary and secondary rainbows. In the primary
rainbow the violet colour is on the inner arc and
the red colour is on the outer arc. In the
secondary rainbow, the sequence of colours is
opposite due to two internal reflections inside
the rain drops.
Secondary
Primary
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ATMOSPHERIC REFRACTION
Refraction of light by earths atmosphere is
called 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.
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Twinkling of Stars 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
earths 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.
Apparent position of the Star
Real position of the Star
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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.
Apparent position of the Sun
Atmosphere
Real position of the Sun
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SCATTERING OF LIGHT BY TINY WATER DROPLETS IN THE
MIST
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SCATTERING OF LIGHT BY SMOKE COLLOIDAL PARTICLES
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SCATTERING OF LIGHT - Activity
Conc. Sulphuric acid
L1
L2
I
Sodium thio sulphate solution (hypo)
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SCATTERING OF LIGHT
Tyndall Effect
The earths 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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Scattering of Light Blue colour of the sky and
Reddish appearance of the Sun at Sun-rise and
Sun-set
Less Blue colour is scattered
Atmosphere
Other colours mostly scattered
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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 a greater distance in the Earths
atmosphere than does 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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