The Human Eye and The Colorful World ||Class 10th Science||CBSE Notes || Study World
The Human Eye The human eye is one of the valuable and sensitive sense organ. It enables us to see the wonderful world and colours around us. Of all the sense organ, the human eye is the most significant organ.
Construction of the eyes:
The human eye is like a camera. The front part of the eye is called Cornea. It
is made of a transparent substance and it is building outward. The light coming
from the objects enter the eye through the cornea.
Just behind the cornea is Iris which control the size of the pupil.
There
is a hole in the middle of the Iris which is called Pupil of the eyes. It look
black because no light is reflected from it.
The Eye-lens is a concave lens made of a transparent soft and flexible material
called like a jelly made of protein. Its focal length can be changed by the
action of ciliary muscles.
The
screen on which the images are formed in the eyes is called retina. The retina
is a dedicated membrane having a large number of lights sensitive cell called rods
and cones.
The
space between cornea and lens is filled with a viscous liquid called "aqueous
humour".
And
the space between the eye-lens and retina is filled with another liquid called "vitreous humour".
Working of the Eye:
The light coming from the object kept in front of us into the pupil of the eye
fall on the eye-lens.
The eye-lens is a convex lens. So it converge the light
rays and produces a real and inverted image of the object on the retina.
The retina has a large number of light
sensitive cell.When the image fall on the retina, then the light sensitive
salary get activated and generate electric signal. The retina sends these
signal to the brain through the optic nerves and give rise to sensation of
vision.
Although
the image formed on the retina is inverted. Our mind interprets the image as
that of an erect object.
Blind Spot:
A small region of the retina where the optic nerve enter the eye ball is insensitive
to light and is called blind spot.
Function of Iris and Pupil.
The
Iris automatically adjust the size of the people according to the intensity of
light received by the eyes. If the amount of light received by the eyes is
greater than pupil contracts (becomes small) and the reduce the amount of light
entering the eye.
On the other hand if the amount of light received by the eye is
small ( as in the dark room ) then the pupil expands
and more light enter into the eyes.
How do we see colour.
The
retina of our eyes has a large number of light sensitive cells. These are the
two types rods shaped and cone shaped.
The
rod shaped cell of the our retina response to the intensity of light.
The
cone shaped cells of our retina respond to the colour.
Accommodation:
An eye can focus the distant image as well as the nearby objects on its retina
by changing the focal length. The ability of the eyes to focus the distant
object at well as nearby object on the retina by changing the focal length of
the lens is called accommodation.
Range of vision of Normal human
eyes:
The
farthest point from the eyes at which an object can be seen clearly is known as
the far point of the eye. The far point of human eyes is infinity.
The
nearest point up to which the eyes can
see the object clearly without any strain is called near point of eye.
The
near point of a normal human eyes is at a distance of 25 centimetre.
Defect of vision and their
correction:
The
ability to see is called vision. It is also called eyesight. Sometime the eye
of a person cannot focus the image of the object on the retina property. In
such a case the vision of the person become blurred and he cannot see either
the distant object or nearby object clearly and comfortably. The person is said to have a defect of vision.
There are three
common defects of vision.
(1) Myopia
(short sightedness or near sightedness)
(2) Hypermetropia
( long sightedness or far sightedness)
(3) Presbyopia.
These
are the refractive defect of vision
because they are caused by the incorrect refraction of light rays by the eye-lens.
Myopia:
Myopia is that defect of vision due to which a person cannot see the distant
objects clearly. E.g. a child having a
defect cannot read the writing on blackboard clearly if he/ she on the last
bench. The far point of the eye suffering from myopia is less than infinity.
Cause of Myopia: This
defect is caused
(a) due to high converging power of eye-lens , or
(b) due to eye ball being too large/ long.
Correction:
Myopia is corrected by using spectacles containing concave lenses. The whole
purpose of using a concave lens here is to reduce the converging power of the eye
lens.
Correction
: Myopia is corrected by using concave lens spectacles.
In Figure, Correction of Myopia: The concave lens placed in front of the eye forms a virtual image of distant object at far point(F) of the myopic eye.
Sample problem:
The
far point of a myopic person is 80 cm in front of the eyes. What is the nature
and power of the lens required to correct the defect.
Solution: Given
Putting these value in lens formula
Hypermetropia:
Hypermetropia is the defect of vision due to which a person cannot see the
nearby objects clearly through we can see distant objects clearly.
The
near point of hypermyopia eye is more than 25 cm away. Hypermetropia is just
the opposite of myopia.
Causes of hypermetropia:
The defect of hypermetropia is called :
(a) due to low converging power of eye lens.
(b) due
to eye ball being too short.
In figure: In hypermetropic eye, the image of nearby object lying at normal near point N (at 25 cm) is formed behind retina.
Presbyopia:
Presbyopia is that defect of vision due to which an old person cannot see the
nearby objects clearly due to loss of the power of accommodation of eye.
Presbyopia
occur in old age due to the gradual weakness of cilliary muscle and diminished flexibility
of eye lens.
Presbyopia
defect is corrected in the same way as Hypermetropia by using spectacles having
convex lens.
Cataract
: The medical condition in which the lens of the eye of a person become
progressively cloudy resulting blurred vision is called cataract.
Glass
Prism
A
triangular glass prism is a transparent object made of glass having two
triangular end and three rectangular sides.
The
refraction of light on passing through a glass prism with different from that
in glass slab. This is because in refraction through a glass slab the emergent
ray is parallel to the incident ray but in refraction through a glass prism, the
emergent ray is not parallel to the incident ray.
Refraction
of light through a glass prism
When
a ray of light passes through a glass prism refraction of light occur both. When
it leave the prism as well as when it leave the prism. Since the refraction surface PQ and PR are not parallel
to one another. Therefore the emergent ray and the incident ray are not parallel
to each other.
In
figure a glass prism PQR has kept on its base QR. A ray of is light is incident on face PQ. As ray AB
goes from rare to denser medium.
So
it bend toward show it bend toward the
normal BN and goes along the direction of BC.
Now
when the ray BC strikes the force PR refraction take place again. Since the ray
BC is going from glass (denser) to air
(air)
So
it bend away from the normal MC and
goes along the direction CD in the form of emergent ray. From the above
discussion, we conclude that when a Ray of light passes through the prism it
suffer two time refraction.
Dispersion of light
In
the year 1665, Newton discovered by his experiment with glass prism that white
light consists of a mixture of seven colour.
The
splitting of white light into seven colour on passing through a transparent medium like a glass prism is called Dispersion
of light.
The
band of seven colours formed on a white light is passed through a glass prism
is called Spectrum of light.
The
seven colour of the spectrum can be denoted by word VIBGYOR.
Rainbow
The
rainbow is an arch of seven colour visible in the sky which is produced by the dispersion
of Sunlight by raindrop in atmosphere.
A
rainbow is produced by the dispersion of white sunlight by rain drops in the
atmosphere. Each rain drop acts as a tiny glass Prism splitting the sunlight into a spectrum.
The formation of seven colours of rainbow in the sky show that white sunlight
consists of a mixture of seven colour lights.
Atmospheric
Refraction
The
refraction of light caused by the earth's atmospheric is called atmospheric refraction.
Under
normal circumstances the air in the upper atmosphere is optically rarer and as
we come down, the air in the lower
atmosphere is optically denser. This arrangement of optical densities of air in
the atmosphere can, however change according to the local conditions such as
temperature at a particular place. Due to this many optical phenomena occur in
the atmosphere.
Twinkling
of Stars.
We
know that star emit their own light due to this, the star shine in the night
sky. At
some moment the stars appears to be very bright and next moment it become very
dim. The twinkling of stars is due to the atmospheric refraction of star’s
light. The continuously changing atmosphere refracts the light from the star by
different amount from one moment to the next. In this way, the star light reaches
our eyes increase and deceases continuously due to the atmosphere refraction
and the star appear to twinkle at night.
As
we know that star are very far off from us. So, star can be considered to be a
point source of light. The continuously
changing atmosphere is able to cause
variation in the light coming from a point size star due to refraction because
of which the stars appear to be twinkle.
On
the other hand, planets appear to be quite bog to us (because they are much
nearer to earth) So, a planet can be considered to a collection of very large
number of point source of light. The damping effect produced by some of the
point source of light in its one part is nullified by the brighter effect
produced by the point source of light in the other part.
Thus on the whole the brightness of the planet always remain the
same and hence it does not appear to Twinkle.
We
can say that the continuous changing
atmosphere is unable to cause variation in the light coming from a big
size planet (due to refraction) because of the planet does not Twinkle.
Stars
seem higher than they actually are
Light
from a Star is reflected (bend) as it leave space (a vacuum) and enter the
earth’s atmosphere. Air higher up in the sky is rarer but that nearer the Earth’s
surface is denser. So, as the light from a star come down, the dense air bends the
light more. Due to this refraction of star's light the star appear to be at a
higher position.
Advance
Sunrise and Delayed Sunset.
When
the sun is slightly below the horizon, then the sun's light coming from less dense
air to more dense air is reflected downward as it passes through the atmosphere.
Because of this atmospheric refraction the sun appear to be raised over the
Horizon when actually it is slightly blow the horizon.
For example in figure the
actual position of sun is at A just below the horizon but it appears to be at
position B above the horizon.
It
is also due to atmospheric refraction that we can still see the sun for above 2
minutes even after the sun has set below horizon.
Scattering
of Light
Scattering
of light means to throw light in various random directions.
Light is scattered when it falls on various type of
suspended particles in its path. The blue colour of sky and the red colour of
the sun at sunrise and sunset can be explained on the basis of scattering of
light caused by the earth’s atmosphere.
Tyndall
Effect
The
scattering of light by particle in its path is called tyndall effect.
When
a beam of Sunlight enter a dusty room through a window, than its path become
visible to us. This is because the tiny dust particles present in the air of
room scatter the beam of light all around the room.
Tyndal
discovered that when that light consists of seven colours passes through a clear
liquid having small suspended particle in it, then the blue colour of white
light having shorter wavelength is scattered more than then the redcolour
having longer wavelength. In fact the blue light present in sunlight is
scattered 10 time more than the red light.
Why
the sky is blue.
The
scattering of blue component of white sunlight by air molecules present in the
atmosphere called the blue colour of the sky.
As
we know that sunlight is made up of seven colour light mixed together when sunlight
passes through the atmosphere most of the longer wavelength light (such as Red,
Orange, Yellow,etc) present in it do not
get scattered much by the air molecules and passes pass straight through.
The
shorter wavelength blue light is however, scattered
all around the sky by air molecules in the atmosphere,whichever direction we
look, some of this scattered blue light enter our eyes. Since we see the blue
sky light from everywhere overhead, the sky look blue.
Note:
If the earth had no atmosphere consisting of there would be no scattering of sunlight
at all.
👉In
the outer space, the sky look dark and black instead of blue. This is because
there is no atmosphere containing air in the outer space to scatter sunlight.
Why
the sun appear Red at Sunrise and Sunset.
The
sun and the surrounding Sky appear red at sunrise and sunset because at that
time most of the blue colour present in sunlight has been scattered out and
away from our line of sight leaving behind mainly red colour in the direction
direct sunlight beam that receive our eyes.
Why
the sun appear white when it is overhead in
the sky.
When
the sun is overhead (as at noon) then the light coming from the sun has to travel a
relatively shorter distance through the atmosphere to reach us.
During this
shorter journey of Sunlight, only a little ofblue colour of the white light is
scattered. Most of the blue light colour remain in it. Since, the light coming
from the overhead sun has almost all the component colour in the right
proportion. Therefore the sun in the sky appear white to us.
In Figure: The scattering of a light of blue light present in white sunlight by the air molecules in the atmosphere makes the sky blue.
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