Light

LIGHT
An important part of our lives

 When we are present in a dark room, we are
not able to see any object.
 Whenever we look a playground at late night,
nothing is visible.
 What are the reasons behind it?
 Why can’t we see objects in dark places?
 LET US FIND OUT.

Light usually refers to visible light, which is electromagnetic
radiation that is visible to the human eye and is responsible for the
sense of sight. Visible light is usually defined as having a wavelength in
the range of 400nanometres (nm), or 400×10−9 m, to 700 nanometers –
between the infrared, with longer wavelengths and the ultraviolet, with
shorter wavelengths.
The main source of light on Earth is the Sun. Sunlight provides the
energy that green plants use to create sugars mostly in the form of
starches, which release energy into the living things that digest them.
This process of photosynthesis provides virtually all the energy used by
living things. Historically, another important source of light for humans
has been fire, from ancient campfires to modern kerosene lamps. With
the invention of electricity, electric lighting has all but replaced firelight.
THE REASON BEHIND IT.

 T H E A N G L E O F I N C I D E N C E ( D E N O T E D A S
( ∠ I ) ) A N D T H E A N G L E O F R E F L E C T I O N
( D E N O T E D A S ( ∠ R ) ) A R E A L W A Y S E Q U A L .
 T H E I N C I D E N T R A Y , T H E R E F L E C T E D R A Y
A N D T H E N O R M A L A T T H E P O I N T O F
I N C I D E N C E A L L L I E O N T H E S A M E P L A N E .
LAWS OF REFLECTION

1ST LAW
•Fix a white sheet of paper on a drawing board or a table.
•Take a comb and close all its openings except one in the middle.
You can use a strip of black paper for this purpose.
• Hold the comb perpendicular to the sheet of paper. Throw light
from a torch through the opening of the comb from one side (Fig.
below).
•With slight adjustment of the torch and the comb you will see a
ray of light along the paper on the other side of the comb.
•Keep the comb and the torch steady. Place a strip of plane mirror
in the path of the light ray (Fig. 16.1). What do you observe?

2ND LAW
•Perform previous activity again. This time use a sheet of
stiff paper or a chart paper.
•Let the sheet project a little beyond the edge of the
Table(Fig. below).
•Cut the projecting portion of the sheet in the middle. Look
at the reflected ray. Make sure that the reflected ray extends
to the projected portion of the paper.
•Bend that part of the projected portion on which the
reflected ray falls. Can you still see the reflected ray?
• Bring the paper back to the original position. Can you see
the reflected ray again?
• This infers the second law of reflection as the reflected ray
does not fall on the bended part of paper.

IMAGE FORMED BY A PLANE MIRROR
• A source of light O is placed in front of a plane mirror PQ. Two rays OA
and OC are incident on it (Fig. below).
• Draw normals to the surface of the mirror PQ, at the points A and C.
• Then draw the reflected rays at the
points A and C. Call the reflected
rays AB and CD, respectively.
• Extend them backwards. They meet. Mark this point as I. For a viewer’s
eye at E , do the reflected rays appear to come from the point I.
• Since the reflected rays do not actually meet at I, but only appear to do
so, we say that a virtual image of the point O is formed at I.
• Such an image cannot be obtained on a screen.
• In an image formed by a mirror the left
of the object appears on the right and the
right appears on the left. This is known
as lateral inversion.

Imagine that parallel rays are incident on an irregular surface as
shown in Fig. a. Remember that the laws of reflection are valid
at each point of the surface. Use these laws to construct
reflected rays at various points. Are they parallel to one
another? You will find that these rays are reflected in different
directions. (Fig. b)
(b)(a)

When all the parallel rays reflected from a plane surface
are not parallel, the reflection is known as diffused or
irregular reflection. Remember that the diffused
reflection is not due to the
failure of the laws of reflection. It is caused by the
irregularities in the reflecting surface, like that of a
cardboard.
On the other hand reflection from a smooth surface like
that of a mirror is called regular reflection.
Images are formed by regular reflection.

 Nearly everything you see around is seen due
to reflected light. Moon, for example, receives
light from the sun and reflects it. That’s how we
see the moon.
 The objects which shine in the light of other
objects are called illuminated objects.
 There are other objects, which give their own
light, such as the sun, fire, flame of a candle
and an electric lamp. Their light falls on our
eyes. That is how we see them.
 The objects which emit their own light are
known as luminous objects.

REFLECTED LIGHT CAN BE
REFLECTED AGAIN
 Recall the last time you visited a hair dresser.
He makes you sit in front of a mirror. After your
hair cut is complete, he places a mirror at your
back to show you how the hair has been cut.
Can you think how you could see the hair at the
back of your head? This marks the way of
showing of images through a periscope.
 The periscope makes use of two plane mirrors.
 Periscopes are used in submarines, tanks and
also by soldiers in bunkers to see things
outside.

Multiple Images
You are aware that a plane mirror forms
only a single image of an object. What
happens if two plane mirrors in
combination are used? Let us see.

Activity to show multiple images
formed by a plane mirror
• Take two plane mirrors.
• Set them at right angles to each other with their
edges touching (Fig. below).
• To hinge them you can use adhesive tape. Place a
coin in between the mirrors. How many images
of the coin do you see
(Fig. below)?

• Now hinge the mirrors using the adhesive tape at
different angles, say
45º, 60º, 120º, 180º etc.
• Place some object (say a candle) in between them.
Note down the number of images of the object in each
case.
• Finally, set the two mirrors parallel to each other.
Find out how many images of a candle placed between
them are formed (Fig. below).

Conclusion
Can you now explain how you can
see the back of your head at the hair
dresser’s shop?
This idea of number of images formed
by mirrors placed at an angle to one
another is used in a kaleidoscope to
make numerous beautiful patterns. You
can also make a kaleidoscope yourself.

Making of kaleidoscope
• To make a kaleidoscope, get three rectangular mirror strips about
15cm long and 4 cm wide each.
• Join them together to form a prism as shown in Fig.(a).
• Fix them in a circular cardboard tube or tube of a thick chart paper.
Make sure that the tube is slightly longer than the mirror strips.
• Close one end of the tube by a cardboard disc having a hole in the
centre, through which you can see [Fig.(b)].
• To make the disc durable, paste a piece of transparent plastic sheet
under the cardboard disc. At the other end, touching the mirrors, fix
a circular plane glass plate [Fig. 16.12(c)].
• Place on this glass plate several small pieces of coloured glass
(broken pieces of coloured bangles).
• Close this end of the tube by a ground glass
plate.
• Allow enough space for the colour pieces
to move around.

Introduction to Kaleidoscope
Your kaleidoscope is ready.When you peep
through the hole, you will be able to see a
variety of patterns in the tube.
Interesting feature of a kaleidoscope is
that you will never see the same pattern
again. Designers of wallpapers and fabrics
and artists use kaleidoscopes to get ideas for
new patterns.To make your toy attractive,
you can wrap the kaleidoscope in a coloured
paper.

Sunlight — White or
Coloured
In ClassVII, we learnt that the sunlight is
referred to as white light.We also learnt that
it consists of seven colours.
Here is another activity (next Activity)
showing that sunlight consists of several
colours.

Dispersion of light
 Get a plane mirror of suitable size.
 Place it in a bowl (Katori) as shown in Fig. (a).
 Fill the bowl with water.
 Put this arrangement near a window such that direct sunlight falls on the mirror.
 Adjust the position of the bowl such that the reflected light from the mirror falls
on a wall. If the wall is not white, fix a sheet of white paper on it.
 Reflected light will be seen to have many colours. How can you explain this?
 The mirror and water form a prism.This breaks up the light into its colours, as we
 learnt in ClassVII.
 Splitting of light into its colours is known as dispersion of light. Rainbow is a
 natural phenomenon showing dispersion.
(a)(b)

We see things only when light coming from them enters our eyes.
Eye is one of our most important sense organs. It is, therefore,
important to understand its structure and working.
The eye has a roughly spherical shape. Outer coat of the eye is
white. It is tough so that it can protect the interior of the eye
from accidents. Its transparent front part is called cornea. (Fig.
below). Behind the cornea, we find a dark muscular structure
called iris. In the iris, there is a small opening called the pupil.
The size of the pupil is controlled by the iris. The iris is the part
of that eye which gives it its distinctive colour. When we say that
a person has green eyes, we refer actually to the colour of the
iris. The iris controls the amount of light entering into the eye.
Let us see how.

 Look into your friend’s eye. Observe the size
of the pupil.
 Throw light on her eye with a torch. Observe
the pupil now.
 Switch off the torch, and observe her pupil
once again. Do you notice any change in the
size of the pupil?
 In which case was the pupil larger? Why do
you think it was so?
 In which case do you need to allow more
light in the eye, when the light is dim or
bright?

Behind the pupil of the eye is a lens which is
thicker in the centre. What kind of lens is
thicker at the centre?
Recall what you learnt about lenses in Class
VII. The lens focuses light on the back of the
eye, on a layer called retina(Fig. 16.14).
Retina contains several nerve cells.
Sensations felt by the nerve cells are then
transmitted to the brain through the optic
nerve.
There are two kinds of cells
(i) cones, which are sensitive to bright
light and
(ii) rods, which are sensitive to dim light.
Besides, cones sense colour. At the junction
of the optic nerve and the retina, there are
no sensory cells, so no vision is possible at
that spot. This is called the blind spot. Its
existence can be demonstrated by the
following activity:

Activity
Make a round mark and a cross on a sheet of paper with the spot to
the right of the cross (Fig. below). The distance between two marks
may be 6-8 cm. Hold the sheet of paper at arms length from the eye.
Close your left eye. Look continuously at the cross. Move the sheet
slowly towards you, keeping your eye on the cross. What do you
find? Does the round mark disappear at some point? Now close your
right eye. Look at the round mark now and repeat the activity. Does
the cross disappear? The disappearance of the cross or the round
mark shows that there is a point on the retina which
cannot send messages to the brain when light falls on it.

The impression of an image does not vanish immediately from the
retina. It persists there for about 1/16th of a second. So, if still images
of a moving object are flashed on the eye at a rate faster than 16 per
second, then the eye perceives this object as moving.
Perform the following activity to understand the concept.
Get a square piece of cardboard of side 6-8 cm. Make two holes as
shown in Fig. below. Thread a string through the two holes.
Draw/paste a cage on one side of the cardboard and a bird on the
other side. Twist the string and make the card twirl rapidly. Do you see
the bird in the cage?
The movies that we see are
actually a number of separate
pictures in proper sequence. They
are made to move across the eye
usually at the rate of 24 pictures
per second (faster than 16 per
second). So, we see a moving
picture.

Important Facts
Nature has provided eyes with eyelids to protect from any
object entering the eye. Eyelids also shut out light when not
required.
Eye is such a wonderful instrument that it (normal) can see
distant objects as well near objects clearly. The minimum
distance at which the eye can see objects distinctly varies
with age. The most comfortable distance at which one can
read with a normal eye is about 25 cm.
Some persons can see near objects clearly but cannot see
distant objects so clearly. On the other hand, some
persons cannot see near objects clearly but they can see
distant objects quite well. With suitable corrective lenses,
these defects of the eye can be corrected.
Sometimes, particularly in old age, eyesight becomes
foggy. It is due to the eye lens becoming cloudy. When it
happens, persons are said to have cataract. There is a loss of
vision, sometimes extremely severe. It is possible to treat
this defect. The opaque
lens is removed and a new artificial lens is inserted. Modern
technology has made this procedure simpler
and safer.

Care of the Eyes
 It is necessary that you take proper care of your eyes. If there is any problem you should
go to an eye specialist. Have a regular checkup.
 If advised, use suitable spectacles.
 Too little or too much light is bad for eyes. Insufficient light causes eyestrain and
headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can
injure the retina.
 Do not look at the sun or a powerful light directly.
 Never rub your eyes. If particles of dust go into your eyes, wash your eyes with clean
water. If there is no improvement go to a doctor.
 Wash your eyes frequently with clean water.
 Always read at the normal distance for vision. Do not read by bringing your book too
close to your eyes or keeping it too far.
 One should include in the diet components which have vitamin A. Raw carrots, broccoli
and green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs, milk,
curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A.

Animals have eyes shaped in different ways. Eyes of a crab are quite small but
they enable the crab to look all around. So, the crab can sense even if the enemy
approaches from behind. Butterfly has large eyes that seem to be made up of
thousands of little eyes (Fig. below). It can see not only in the
front and the sides but the back
as well.
A night bird (owl) can see very well
in the night but not during the
day. On the other hand, day light
birds (kite, eagle) can see well
during the day but not in the
night. The Owl has a large cornea
and a large pupil to allow more
light in its eye. Also, it has on its
retina a large number of rods and
only a few cones. The day birds
on the other hand, have more
cones and fewer rods.

Some persons, including children, can be visually handicapped. They have very limited vision to
see things. Some persons cannot see at all since birth. Some persons may lose their eyesight
because of a disease. Such persons try to identify things by touching and listening to voices
more carefully. They develop their other senses more sharply. However, additional resources can
enable them to develop their capabilities further.
Resources can be of two types : Non-optical aids and optical aids.
Non-optical aids include visual aids, tactual aids (using the sense of touch), auditory aids (using
the sense of hearing) and electronic aids. Visual aids, can magnify words, can provide suitable
intensity of light and material at proper distances. Tactual aids, including Braille writer slate and
stylus, help the visually challenged persons in taking notes, reading and writing. Auditory aids
include cassettes, tape recorders, talking books and other such devices. Electronic aids, such as
talking calculators, are also available for performing many computational tasks. Closed circuit
television, also an electronic aid, enlarges printed material with suitable contrast and
illumination. Nowadays, use of audio CDs and voice boxes with computers are also very helpful
for listening to and writing the desired text.
Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids.
While the lens combinations are used to rectify visual limitations, telescopic aids are available to
view chalkboard and class demonstrations.

The most popular resource for visually challenged persons is
known as Braille.
Louis Braille, himself a visually challenged person, developed
a system for visually challenged persons and published it in
1821.
Braille system has 63 dot patterns or characters. Each
character represents a letter, a combination of letters, a
common word or a grammatical sign. Dots are arranged in
cells of two vertical rows of three dots each.
The present system was adopted in 1932. There is Braille
code for common languages, mathematics and scientific
notation. Many Indian languages can be read using the
Braille system. Patterns of dots to represent some English
alphabets and some common words are shown below.
These patterns when embossed on
Braille sheets help visually
challenged to recognize words by
touching. To make them easier to
touch, the dots are raised slightly.

Visually challenged people learn the Braille system by beginning with
letters, then special characters and letter combinations. Methods depend
upon recognition by touching. Each character has to be memorized. Braille
texts can be produced by hand or by machine. Type writer - like devices and
printing machines have now been developed.
Some visually challenged Indians have great achievements to their
credit. Diwakar, a child prodigy has given amazing performances
as a singer.
Mr. Ravindra Jain, born completely visually challenged,
obtained his Sangeet Prabhakar degree from Allahabad.
He has shown his excellence as a lyricist, singer and
music composer.
Mr. Lal Advani, himself visually challenged, established
an Association for special education and rehabilitation
of disabled in India. Besides, he represented India on
Braille problems to UNESCO.
Helen A Keller, an American author and lecturer, is
perhaps the most well-known and inspiring visually
challenged person. She lost her sight when she was
only 18 months old. But because of her resolve and
courage she could complete her graduation from a
university. She wrote a number of books including
The Story of my Life (1903).

MADE BY:-
Name- Sunny Bagde
Class- VIII-B
Roll no:- 06

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