Human Eye
Human eye is one of the
most valuable and sensitive organs. It resembles a camera. The human eye works
on the refraction of the light through a natural convex lens. Main parts of
human eye :
Cornea
·
Iris
·
Pupil
·
Cillary muscles
·
Eye lens
·
Retina
·
Optic nerve
1) Cornea: The front part of
the eye is called as cornea. It is made of a transparent substance .It is a
thin membrane of eye, through which light enters the eye first and gets
refracted.
2 ) Iris: Iris is suited just
behind the cornea. Iris is a flat, coloured, ring shaped membrane. It control
the size of pupil.
3) Pupil: There is a hole in
the middle of the iris which is called the pupil of the eye. The pupil appears
black because no light is reflected from it. Pupil regulates and controls the
amount of light entering the eye.
4) Cillary muscles : It holds
the eye lens and lens by focusing. The focal length of the convex lens present
inside the eye can be changed by the action of the Cillary muscles.
5) Eye lens: Eye lens is a
double convex lens made of a transparent , soft and flexible material like a
jelly made of proteins . Being flexible the eye lens can change its shape (it
can become thin or thick) to focus light on the retina.
6) Retina : The screen on
which the image is formed in the eye is called retina. The retina is behind the
eye lens. It is a delicate membrane having a large number of light, sensitive
cells called ‘rods and cones’ which response to the intensity of light and
colour of object respectively by generating electrical signals.
7) Optic nerve: It is a nerve
connecting eye and brain. It sends the electrical signals to brain.
Working of Human Eye
The
light rays coming from the object kept in front of us enter through the cornea
of the eye, pass through the pupil of the eye and fall on the eye lens so it
converges the light rays and produces a real and inverted image of the object
on the retina is converged to the brain by the optic nerve and gives rises to
the sensations of vision.
NOTE : Rod and Cone cells: The retina of our eye has
a large number of light sensitive cells. These are two kinds of light sensitive
cells on the retina : rods and cones.
Rods are the rod – shaped cells present
in the retina of an eye which are sensitive to dim light. Rods are the most
important for vision in dim light. Natural animals which sleep during the day
and come out at night like the owl have a large number of rod cells in their
retina which help them to see properly during the night where there is not much
light.
Cones are the cone shaped cells which are
sensitive to bright to bright light for normal light. The cone cells of our
retina also respond to colour.
Power of Accomodation
An
eye can focus the images of the distant object as well as the nearby object on
its retina by changing the focal length (or converging power of its lens. The
focal length of the eye lens is changed by the action of the Cillary muscles).
The Cillary muscles can change the thickness of the soft and flexible eye lens
and hence its focal length which in turn, changes the converging power of the
eye lens.
The ability of an eye to focus
the distant object as well as the nearby objects in the retina by changing the
focal length for converging power of its lens is called accommodation. An
normal eye has a power of accommodation which enables objects as far as
infinity and as close as 25 cm to be focused on the retina.
Far point : the farthest point from
the eye at which an object can be seen clearly is known as the “ far point” of
the eye. The far point of a normal human eye is at infinity.
Near point: The minimum distance at
which an object must be placed so that a normal eye may see it clearly without
any strain, is called the least distance of distinct vision. The least distance
of distant vision for a normal eye may see it clearly without any strain, is
called the least distance of distant vision for a normal eye is about 25 cm .
Note: colour combination:
Red
+ green= yellow
Blue
+ green = Peacock blue
Red
+ Blue= Magenta
Defect of vision
Sometimes
the eye of a person cannot focus on the retina properly. In this case the person is said to have a
defect of vision . There are three common defects of vision.
Myopia:
Myopia ( or short – sightedness ) is
that defect of vision due to which a person cannot see the distant objects
clearly through he can see the nearby objects clearly. The far point of an eye
suffering from myopia is less than infinity.
Causes of myopia
The
defect of eye called myopia (for short – sightedness) is caused
1)
Due to high converging power of eye lens.
2)
Due to eye ball being too long.
Correction of myopia
Myopia
(short –sightedness or near –sightedness ) is corrected by using spectacles
containing concave lenses
Diagram
Hypermetropia
Hypermetropia
or ( long – sightedness) is that defect of vision due to which a person cannot
see the nearby objects clearly though he can see the distant objects clearly.
The near point of a hypermetropia is centimetres away from retina.
Causes
of Hypermetropia
1 Due to low converging power of eye
lens.
2 Due to eye – ball being too short.
Correction of hypermetropia
Hypermetropia
( long – sightedness or far – sightedness) is corrected by using spectacles
combining convex lenses.
Diagram
Presbyopia
Presbyopia
is that defect of vision due to which an old person cannot see nearby objects
clearly due to the loss of power of accommodation of the eye.( The near point
in this case become less than 25 cm ) .
Cause
1
Due to Cillary muscles becoming weak
2
Due to the eye lens becoming rigid
Correction
Presbyopia
is corrected by using spectacles containing cylindrical lens.
There
are few modes of defect of vision
Cataract
It is a defect of vision in which usually
comes in oblage is cataract. The medical condition in which the lens of the
lens of eye of a person becomes progressively resulting in blurred vision is
called cataract.
Cause
Cataract
develop when the eye lens of a person becomes cloudy due to the formation of a
membrane over it.
Correction
The
opaque lens is removed from the ye of
the person by surgical operation and a new artificial lens is inserted in the
place.
Colour blindness
It
is a genetic disorder caused due to the absence of cone cells in the retina. In
this defect, vision is normal, but eye cannot between colours.
Q Why do we have 2 eyes for vision
and not just one?
Ans
There are many advantages of having two eyes instead of one. Some of them are
given below:
1. Having two eyes gives a wider field of view
2. Having two eye enables us to judge distance
more accurately.
Q What is night blindness?
Ans.
Some persons have the difficulty to see the objects in dim light during night.
This defect of eye is called night blindness.
Cause
1
Lack of vitamin a in the food
2
Improper functioning of rods – shaped cells. The rods shaped cells respond to
intensity variation OF LIGHT.
Q What is total internal reflection ?
Ans When light passes from an optically denser medium
to a rarer medium beyond certain angle of incidence it undergoes reflection
instead of refraction. This is called total internal reflection and that
certain minimum angle of incidence is called critical angle condition for total
internal reflection.
1
Light should pass from denser to rarer medium.
2
Angle of incidence should be greater than critical angle.
Atmospheric refraction
: The
refraction of light by the Earth’s atmosphere is called as atmospheric
refraction.
Twinkle stars
The twinkling of stars is due to the
atmospheric refraction of starlight. The starlight on entering the Earth’s
atmosphere undergoes refraction continuously before it reaches the Earth. The
continuously changing atmosphere refracts the light from the stars by different
amounts from one moment to the next. When the atmosphere refracts more star
light toward us, the star appears to be bright and when the atmosphere refracts
less star light, then the star appears to be dim. In this way, the star light
reaching our eyes increases and decreases continuously due to atmospheric
refraction and the star appears to twinkle at night.
· Advance sunlight and delayed sunset
Sun
is visible to us about 2 min after the actual sunset because of atmospheric
refraction. By actual sunset because of atmospheric refraction. By actual
crossing of the horizon by the sun. The time difference between the actual
sunset and the apparent sunset is about 2 min.
Rayleigh law of scattering
According
to ray light, the amount of scattering is inversely proportional to the fourth
power of the wavelength . Therefore the light of shorter wave length is
scattered much more than the light of longer wavelength. There is no change in
the wavelength of light rays during scattering.
Note: thE MOST BEAUTIFUL PHENOMENA OF
NATURE SUCH AS BLUE COLOUR OF SKY. White colour of clouds, Red of sunrise and
sunset can be explained in terms of scattering of light.
Blue colour of sky
Blue
colour have a shorter wavelength than red. So according to Ray light scattering
law, Blue colour of sunlight scattered much more strongly by the larger no. Of
molecules present in the Earth’s atmosphere. Hence the sky appears blue.
Mirage
The apparent inverted images formed due to not
surface in deserts is called mirage. This is caused by total reflection of
light in atmosphere.
TYNDALL EFFECT
The
phenomenon of scattering of light by the colloidal particles known as Tyndall
effect. This effect can be observed when;
1
A fine beam of sunlight enters a room containing suspended
particles of dust, the path of the beam of light visible .It is due to the
scattering of light.
2
Sunlight passes through a canopy of dense forest, mist
contains tiny droplets of water which acts as particles of colloid dispersed in
air.
Dispersion of light
In the year 1665 Newton found that if
a beam of white light is passed through a triangular glass prism the white
light splits to form a band of seven colours. The splitting of white light into
its constituents colours is called dispersion.
The band of seven colours formed on
a white screen, when a beam of white light is passed through a glass prism, it
is called spectrum of white light. The seven colours of the spectrum are red,
orange, yellow, green, blue, indigo and violet.
Note: When the light rays change its path
due to some particles it is termed as Scattering of light.
Causes of Dispersion : The dispersion of white light occurs because colour of white
light travels at different speed through a glass prism. When white light consisting
of seven colour falls on a glass prism, each colour in it is refracted by different angles, with the result that seven
colours are spread out to form a spectrum.
Ques. How rainbow is formed ?
Ans. A Rainbow is produced by the
dispersion of white (sun) light by raindrops in the atmosphere. Each raindrop
acts as a tiny glass prism splitting the sunlight into a spectrum. A Rainbow is
always formed in a direction opposite to
that of the Sun.
Ques. Why does not glass prism produce a dispersion of light?
Ans. A glass slab acts as a
combination of two identical glass prism is in an inverted position with
respect to the first. The first prism splits the white light into the seven
colours. The colour components fall on the second prism, where it recombines
them to form white light.
Ques. Why does the sun appear reddish early in the morning?
Ans. At sunset or sunrise, the sun
rays pass through a maximum length of atmosphere. Much of the blue light and
shorter wave length has been taken out by scattering. Only the red colour of
light reaches the observer. That is why when sunset and sunrise appear red.
Ques. Explain why the planets do not twinkle?
Ans. The planets are much closer to
the Earth and are thus seen as extended sources. If we consider a planet as a
collection of a large number of point sized sources of light, the total
variation in the amount of light entering our eye from all individual point
-sized sources will average out to zero thereby nullifying twinkling effects of
planets.
Ques. Why does the sky appear dark instead of blue to an
astronaut?
Ans. The sky appears dark instead of
blue to an astronaut because at such a huge height there is no atmosphere to
scatter the light. Hence the sky appears completely dark.
