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187 Refraction

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Class 10th
Physics THE HUMAN EYE AND REFRACTION
STRUCTURE OF HUMAN EYE

An eye is the most important organ of the human body. It is associated with vision and acts as a vital
optical instrument. Human beings contain two eyes embedded in the eye sockets. It is spherical in shape and is
covered externally by a hard thick and opaque layer called Sclerotica. The human eye is the most important
organ. It is used to see he beautiful nature and the natural phenomena. The main parts of an eye and their
functions are given below:

1. Cornea:-
Cornea:- It is transparent part of eye and allows the light to enter in the eye. Cornea is also known as the
window or aperture of the eye.

2. Iris:-
Iris:- It is a circular diaphragm having a hole in its centre .this hole is called pupil. The circular
diaphragm has muscles and coloured pigments. The colour of an eye depends upon the colour of these
pigments. The function of iris is to control the amount of light enlarging the eye through the pupil. The
pupil becomes small when bright light falls on the eye. However, it becomes wide when there is dim
light.

3. Lens:-
Lens:- The eye lens is a crystalline double convex lens and made of transparent and flexible tissues. It is
behind the pupil and held by the muscles called citify muscles. It focuses the images of objects on the
retina of the eye.

4. Ciliary muscles:
muscles:- These muscles hold the eye lens in position. Ciliary muscles control the focal length of
the eye lens. When these muscles contract, the focal length of the lens increases. On the other han d,
when they expand, they exert inward pressure on the eye lens and decrease the focal length of the lens.

5. Retina:-
Retina:- It acts as a screen to obtain the image of the object. It contains number of cells in the form of
rods and cones which are sensitive to light. These cells convert light energy into nerve impulses or
signals.

6. Optic nerve:-
nerve:- Optic nerve is formed by the nerve Libras coming from the retina. It carries nerve
impulses or signals to the brain .the brain fi8nally interprets the signal.

WORKING OF THE EYE

The light rays reflected from object surfaces


fall on the exposed portion of the eyes. These rays
enter into the eye through the pupil and falls on the
convex lens, or eye lens. It converges the incident
light rays and forms a real and inverted image of the
object on the yellow spot of ht Retina. The image
formed on the Retina is then converted into nerve
impulses and send to brain through the optic nerve,
which gives rise to the sensation of vision. The
sensation of vision produced is correct and
complete and shown no effect of the invert image
formed on the Retina. However, the amount of
incident light rays entering into the eye is regulated
by the iris. By adjusting the size of its pupil. If the
amount of light rays incident on the eye is high. The
pupil decreases its size and thus reduces the
intensity of light and if the intensity of the incident
light is low the size of the pupil is increased so as to
facilitate entrance of sufficient amount of light into. Diagram Showing Structure of Human Eye.
Compiled By: Riyaz Kathjoo (Dean Academic Affairs GVEI) Comp. Editor: Hilala Jan
Page 2
Class 10th
Physics THE HUMAN EYE AND REFRACTION
ACCOMMODATION

A normal eye can visualize the distant objects as well as a nearby object. This is achieved with the help of
cilliary muscles, which changes the thickness and hence the focal length of the eye lens. Resulting in the alteration
of converging power of our eyes. This ability of an eye to focus the distant objects as well as the nearby objects on
the Retina by changing the focal length or the converging power of its lens is called as Accommodation. The
accommodation power of a normal eye varies in between infinity and 25 cms i.e., a normal eye sis capable to
focus the objects a far as infinity and as close as 25 cms on the Retina.

RANGE OF VISION

The maximum distance within which an eye can focus the various object is called as the range of vision.
It varies in between infinity and 25 cms from an eye i.e., a normal human eye can focus the objects clearly lying
in between infinity and 25 cms from it.

DEFECTS OF VISION

There are various defects which can occur in the human eye like Myopia. Hypermetropia. Presbiopia,
Astigmatism and Cataract etc. the commonly occurring defects are explained as:-

1) Myopia
Myopia Or Short-
Short-sightedness:-
sightedness:- Myopia is commonly occurring defect of the eye which makes it unable
to visualize clearly the distant objects, though it can see nearby objects. Thus defect is due to the high converging
power of the eye lens, which is created by the insufficient relaxation of the cilliary muscles as a result the image of
distant object is formed in front of the Retina.

However the defect can be corrected by using spectacles containing Concave lenses. This diverges the
incident light rays coming from the distant object and helps the lens to focus the image on the right spot of
the Retina and so that the eyes can it clearly.

/
Rays from infinity O I

MYOPIA EYE CORRECTION

2) Hypermetropia or Far- Far-sightedness:-


sightedness:- Hypermetropia is a defect of the eye in which an eye is unable to see
the nearby objects perfectly though it can see the distant objects clearly. This defect is caused due to the low
converging power of the eye lens, which is mainly due to weakening of the cilliary muscles, resulting in the
reduction of the ability of the cilliary muscles. Due to the low converging power of the eye lens, the image of the
nearby object is however, the detect can be corrected by using specials containing convex lenses, which converges
the meident light rays from a nearby object and helps the eye lens in forming the image at the right spot of the
Retina. So that an eye can see it clearly

O I Q

HYPERMETROPIA EYE CORRECTION

3) Presbiopia:
Presbiopia:-- A human eye which cannot see the near object as well as far off (or distant) objects clearly is
said to suffer from a defect known as Presbyopia. This defect arises due to the ageing of a person. The cilliary

Compiled By: Riyaz Kathjoo (Dean Academic Affairs GVEI) Comp. Editor: Hilala Jan
Page 3
Class 10th
Physics THE HUMAN EYE AND REFRACTION
muscles are weakened and the flexibility of the crystalline lens of the human eye decreases with age of the
personal. As a result, human eye is unable to see the near as well as distant objects clearly.

CORRECTION OF PRESBYOPIA

This defect can be corrected by using a bi-focal lens. A bi-focal lens consists of a concave lens which
forms the upper surface of the bi-focal lens and a convex lens which forms the lower surface of the bi-focal lens.
The upper surface of bi-focal lens (i.e the concave lens) enables the person to see distant objects clearly and the
lower surface of the bi-focal lens (i.e., convex lens) enables the person to see the near objects.

DISPERSION OF WHITE LIGHT BY A GLASS PRSIM

When white light passes through a glass prism, we get a band of seven colours on a white screen. The
colours obtained on the screen are violet, indigo, blue, green, yellow, orange and red. Therese colours can be
remembered as VIBGYOR.

From this observation, we can conclude the white light is a mixture of seven colours. The phenomenon
of splitting white light into seven colors 1 known as dispersion.

White light

Prism

VIBGYOR

DISPWESION OF LIGHT

White phenomenon of splitting white light into seven colors when it passes through a glass prism is
called dispersion of white light.

CAUSE OF DISPERSION

White light consists of seven colours: red, orange, yellow, green, blue, indigo and violet. Each colour has
its own wavelength. The wavelength of red colour is the longest and the wave length of violet colour is the
shortest. The speed of light or colour depends upon the wavelength. If wave length of a colour is large, the speed
of the colour is also large. Thus, each colour of light travels with different speeds in a given medium. The speed
of red colour is more than the speed of orange colour in a medium. The speed of orange colour is more than
the speed of yellow colour and so on. Therefore, the speed of red colour in a medium is the highest and the
speed of violet colour is the least. Hence, all colours of white light are refracted by different amount while passing
through the glass prism. Therefore, when a white light passes through a glass prism, different constituent colours
come out of the prism at different angles. This gives rise to the dispersion of white light.

Compiled By: Riyaz Kathjoo (Dean Academic Affairs GVEI) Comp. Editor: Hilala Jan
Page 4
Class 10th
Physics THE HUMAN EYE AND REFRACTION
ATMOSPHERIC REFRACTION

When sum light enters the earth’s atmosphere, it continuously goes from rarer to the denser medium
and hence refraction of light takes place. The refraction of light taking place in the atmosphere is known as
atmospheric refraction.

Atmospheric refraction gives rise to many optical phenomena which are discusses below:

1. Twinkling of stars:-
stars:- light emitted by stares passes through the atmosphere of the earth before reaching
our eyes. The atmosphere of the earth is not uniform but consists of many layer of different densities the layers
close to the surface of the earth are optically denser. As we go higher and higher, the density of layers and
refractive index decreases progressively. As the light from a state enters the upper most layer of the atmosphere,
it bends towards the normal as it enters the next layer. This process continues till the light enters our ryes. So due
to refraction of light, the apparent position of the star is different from the actual position of the star.

Moreover, the different layers of the atmosphere are mobile and the temperature and the density of
layers of atmosphere changes continuously. Hence, the apparent position of the star changes continuously. The
intensity of light coming from the star also changes with the change in the apparent position of the star. This
leads to twinkling of a star. The change in the intensity of light entering our eyes due to the change in hew
apparent position of the star continuously leads to the twinkling of a star.

2) We can see the sun for few minutes even after it has actually set:-
set:- The sun sets when it is below the horizon in
the evening. The rays of light from the sun below the horizon reach our eyes because of refraction of light. These
rays appear to come from the apparent position of the sun which is about the horizon. Hence we can see the sun
fore few minute (about 2 minutes) even after it has actually set. Similarly, the s can be seen about 2 minutes
before it actually rises. Thus, we gain about 4 minutes of additional daylight each day.

ACATTERING OF LIGHT

When sunlight enters the atmosphere of the earth, the atoms and molecules of different gases present in
the atmosphere absorb this light. Hence, these atoms and molecules of the gases re-emit light in all direction.
This process is known as scattering of light. The atoms or particles scattering light are known as scatters.

Q. Why is the colour of clear sky blue?

When sunlight enters the earth’s atmosphere the atoms or molecules of the gases present in the
atmosphere scatter this light. Since wavelength of red colour is larger than the wavelengths of other colours in
sunlight, so red colour is scattered the least. Violet colour is scattered the most followed by blue, green, yellow,
orange and red colours respectively. Our eye is more sensitive to the blue light than the violet light. Therefore,
scattered light in the sky contains blue colour in plenty and hence the clear sky appears blue.

Q. Why sunset and Sunrise are Red?

At the time of sunrise or sunset, the position of the sun is very far away from us. The sunlight travels
longer distance through the atmosphere of the earth before reaching our eyes. Scattering of blue light is more
than the scattering of red light. As a result of this, more red lights reaches our eyes than any other colour. Hence
sunset and sunrise appear red.

Q. An astronaut on the ear


earth looks towards the clear sky. He observes the blue colour of the sky. However,
when he is in space, he observed the sky as dark.
dark. Explain, why?

The sunlight scatters, when enters the earth’s atmosphere. The scattering of light in the earth’s
atmosphere is due to the presence of atoms, molecules. Droplets and dust particles etc. the intensity of scattered
light is more for the colour of less wave length. Since the wavelength of blue colour is less than the wave length of
Compiled By: Riyaz Kathjoo (Dean Academic Affairs GVEI) Comp. Editor: Hilala Jan
Page 5
Class 10th
Physics THE HUMAN EYE AND REFRACTION
red colour, so blue colours scatters more than the red colour. Hence, sky appears blue to an astronaut standing
on the earth.

When the astronaut is in space, then there is no atmosphere (or atoms, molecules, droplets and dust
particles) around him. Therefore, sun light does not scatter and hence sky appears dark.

Q. Explain why planets do not twinkle?

Plants are very close to the earth as compared to the stars. So the intensity of light we receive from the
planet is very large. Therefore, the variation in the brightness of the planets is not detected. Hence, planets do
not twinkle.
Apparent Position of SUN
Observer
Horizon SUN

EARTH Observer

Atmosphere
SUN EARTH

D SUN

Actual Position of SUN

Apparent Position of Star


Actual Position of Star

Sun Light

Violet
Refractive
Index
Red Increasing

Water Drop

Observer
Rainbow formation

Eye

Twinkling of Star

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Compiled By: Riyaz Kathjoo (Dean Academic Affairs GVEI) Comp. Editor: Hilala Jan

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