Download The Human Eye and the colourful world

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Works on the refraction of light through
a natural convex lens made of transparent
living material and enables us to see
things around us.
The main parts of the Human Eye:
Cornea, Iris pupil,ciliary muscles,Eye
lens,Retina and optic Nerve.
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Cornea
The front part of the eye.
Made of a transparent substance and is bulging outwards.
The outer surface is convex in shape.
Light from objects enters the eye through it.
Iris.
Behind the cornea.
A flat,coloured,ring-shaped membrane.
Controls the amount of light entering the eyes.
It automatically adjusts the size of the pupil according to the
intensity of light.
If amount of light large-Iris contracts the pupil and reduces
the amoun t of light.
If light is less- Iris expands the puil so that more light enters
the eyes.
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Pupil.
A Hole in the middle of the Iris.
It appears black because no light is reflected
from it.
Eyelens.
A convex lens made of a transparent,soft
and flexible material like a jelly made of
proteins.
Eyelens can change its shape (thick or thin)
to focus light on the retina.
It is held in position by suspensory
ligaments –one end is connected to the eye
lens and the other to the ciliary muscles.
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Ciliary muscles.
Change the thickness of the eyelens while
focussing.
Main diff between a camera and an eye.
Retina.
The screen on which the image is formed in the
eye.
It is behind the eyelens and at the back part of
the eye.
It is like a film in a camera.
A delicate membrane having a large number of
light-sensitive cells called Rods and cones which
respond to the intensity of light and colour of
objects respectively.
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Blind spot.
A junction of optic nerve and retina in the eye,
where there are no rods and cones.
No vision is possible at this spot.
A small area of the retina insensitive to light
where the optic nerve leaves the eye.
When the image of an object falls on the
blindspot, we cant see the object.
Aqueous Humour
A watery liquid filled between cornea and
Eyelens.
Vitreous Humour
A Transparent jelly-like substance between
eyelens and the retina.
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Rods
Rod-shaped cells in the retina.
Sensitive to dim light.
We are seeing things in dimlight due to the rods.
Nocturnal animals (owl) have a large number of
rods.
Cones
Cone-shaped cells in the retina.
Sensitive to bright light.
Respond to colours, cause the sensation of colour
of objects in our eyes.
To see colours and distinguish between the
colours.
Do not function in dim light.
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The light from the object enters through the
cornea,pass through the pupil and fall on the
eyelens.
Eyelens converges thelight rays and forms a
real and inverted image on the retina.
The image formed in the retina is conveyed
to the brain as electric signals through the
optic nerve and gives rise to the sensation of
vision.The electric signals are generated due
to the activation of rods and cones in the
retina.
Physics concept: Eye- A convex lens and a
screen.
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A normal eye can see the far-away objects and the
near-by objects due to the power of Accommodation.
Distant object view:
The rays of light will be diverging in the beginning
and then becomes parallel.
The convex eyelens should have a low converging
power to focus the image on the retina.
For low converging power,f should be large and is quite
thin.
Near-object view:
The rays of light will be diverging when they reach
the eye.
Convex eyelens -high converging power-low focal
length-lens should be thick.
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The power of Accommodation is possible by changing the focal
length of the eyelens by the action of ciliary muscles.
Distant -object view:
The ciliary muscles of the eye are fully relaxed.
Pull the suspensory ligaments .
The suspensory ligaments will be tight.
The eyelens become stretched and become thin.
Focal length becomes large and converging power becomes low.
In this condition,the eye is said to be unaccommodated because it
is the relaxed state of the eye.
Near-by object view:
Ciliary muscles contract.
Suspensory ligaments loose.
The Eye-lens bulges under its own elasticity and becomes thick.
Focal length less,
Converging power large
In this condition,the eye is said to be accommodated
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The ability of an eye to focus the distant
objects as well as the nearby objects on
the retina by changing the focal length of
its lens is called accomodation.
A normal eye has a power of
accommodation which enables objects as
far as infinity and as close as 25 cm to
be focussed on the retina.
The power of accommodation of normal
vision is 4 dioptres.
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Far point
The farthest point from the eye at which
an object can be seen clearly.
It is at infinity.
Near point,
The nearest point up to which the eye
can see the object clearly.
It is 25 cm.
Therefore,
The range of vision of a normal Human
Eye is from infinity to 25 cm.
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They are also known as the defects of the
eye.some common defects: Myopia,
Hypermetropia,presbyopia.
These are the refractive defects of vision
because they are caused by the incorrect
refraction of light rays by the eye-lens.
Can be corrected by using suitable
spherical lenses.
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Short-sightedness-short sight is normal,but long sight is defective.
Cannot see the distant objects clearly.
The far point is less than infinity.
The causes: High converging power of eyelens.
Or
The Eye ball being too long.
In an eye suffering from Myopia, the ciliary muscles do not relax
sufficiently,to make the lens low converging power.
And the lens thin.
In another case, when the eyeball is too long,the retina is at a larger
distance from the eyelens.
So -the image formed in front of the retina for both the cases.
The defect can be corrected by using concave lens.
The distant object can be made to appear as if it were at the farpoint of the
eye by using a concave lens.
The concave lens should be of such a focal length that it produces a virtual
image of the distant object at the farpoint of the myopic eye.
The purpose of cocave lens is to reduce the converging power of the eyelens.
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Cannot see the near-by objects.
Can see the far-away object.
The nearpoint is more than 25 cm .
Opposite of Myopia.
The causes:
Low converging power of Eye lens (because of large focal length.)
Or
Due to eyeball being too short. The ciliary muscles being weak,
cannot make the eyelens thicker to increase its converging power.
The image is formed behind the retina.
The eye ball short-the retina is at a shorter distance from the
eyelens.
The image is formed behind the retina.
Can be corrected by using convex lens.
Its Focal length should be such that it forms a virtual image of
the object at the nearpoint of the hypermetropic eye.
The purpose of convex lens is to increase the converging power of
the eyelens.
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The defect of vision due to which an older
person cannot see the nearby objects clearly.
In old age, ciliary muscles become weak and
the eyelens become inflexible.
The nearpoint will be much more than 25 cm.
It is old age Hypermetropia.
Caused by the loss of power of
accommodation of the eye due to old age.
It is corrected by convex lens.
Both Myopia and Hypermetropia-upper:
concave, lower: convex
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The medical condition in which the lens of
the eye of a person becomes
progressively cloudy resulting in blurred
vision.
The eyelens becomes cloudy due to the
formation of a membrane over it.
It decreases the vision of the eye
gradually.
Surgery-the eyelens can be removed and a
new artificial lens is inserted.
No spectacle lenses for correction.
Glass prism
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A transparent object made of glass having two triangular
ends and three rectangular sides.
The opposite faces of the prism are not parallel to each
other.They are inclined at an angle to one another.
The angle between its opposite faces is called the angle of
the prism.
For a rectangular slab, the incident ray and emergent ray
are parallel to one another.
For a prism,the incident ray and emergent ray are not
parallel to one another.
In refraction through a prism,the emergent ray is deviated
from its original direction by a certain angle.
The angle between the incident ray produced and the
emergent ray produced is known as the angle of deviation.
Refraction of light through a glass prism
Dispersion of Light
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Newton found that if a beam of white light is passed
through a prism,the white light splits to form a band of
seven colourson a white screen.
The band of seven colours formed on a white screen,when
a beam of white light is passed through a prism is called
spectrum of white light.
This splitting of white light into seven colours is known as
the Dispersion of Light.
Dispersion occurs because the colours of white light travels
at different speeds through a prism.Due to this,each colour
is refracted at a different angle,with the result that seven
colours are spread out to form a spectrum.
Red colour: Maximum speed, Deviation the least.
Forms the upper part of the spectrum.
Violet:Minimum speed,Deviation the max, Forms the lower
part of the spectrum.
Re-combination of spectrum colours to give
white light
Formation of Rainbow
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An arch of seven colours visible in the
sky which is produced by the dispersion
of sun’s light by raindrops in the
atmosphere.
It is actually a natural spectrum of
sunlight in the sky.
It is formed when the sun is shining and
it is raining at the same time.
It is formed in a direction opposite to
the sun.
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It is produced by the dispersion of white
sunlight by the raindrops in the atmosphere.
Each tiny raindrop act as a prism splitting the
sunlight into a spectrum.
A ray of sunlight enters the raindrop and
undergoes refraction and dispersion to form a
spectrum.
This spectrum undergoes total internal reflection
within the raindrop and finally refracted out of
the raindrop.
This spectrum produced by raindrops in the
atmosphere is seen from the earth as Rainbow.
Red at the top,violet at the bottom
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Some of the air layers of the atmosphere are
cold and some are hot.
Hot:optically rarer.
Cold:optically denser.
In the same atmosphere, we have air layers
of different optical densities..
When light rays pass through the air
layers,refraction of light takes place.
The Refraction of light caused by the earth’s
atmosphere having air layers of different
optical densities is known as Atmospheric
Refraction.
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Look at objects through the hot air over a
fire,the objects appear to be moving
slightly.
Air just above the fire: hot: optically rarer.
Above that: cold:optically denserDue to the
Atmospheric refracton,we feel the objects
are moving slightly.small scle
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Twinkling of stars.
Stars seem Higher than they actually are.
Advance sunrise and delayed sunset.
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Star-A point-sized object.
Light coming from the star enters the earth’s
atmosphere.
Due to the varying optical densities of air at various
altitudes,it undergoes Atmospheric Refraction.
The atmosphere is continuously changing.
It refracts the light from the stars by different amounts
from one moment to the next.
When the atmosphere refracts more sunlight towards
us: the star appears to be bright.
When the atmosphere refracts less sunlight towards us:
the star appears to be dim.
The star-light reaching our eyes increases and
decreases continuously due to Atmospheric Refraction.
So the star appears to twinkle.
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A planet-collection of point sources of
light.
The dimming effect produced by some of
the point sources of light is nullified by the
brighter effect produced by the point
sources of light in the other part.
Thus,the brightness of the planet always
remain the sameand hence it does not
appear to twinkle.