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Introduction to Light
• We learn a lot about the world by looking around. How
do we see things? We see an object when light from it
enters our eyes. But what is light? Light is a form of
energy. It can travel from place to place following a few
simple rules. By understanding these rules we can
understand the seven colours of the rainbow, why the
sky is blue, or eclipses of the sun and the moon. In this
chapter we are going to learn some of these beautiful
and strange things that light does when travelling from
one place to the other. We'll also learn about some
instruments that use these simple rules of light to see
faraway stars and tiny creatures that our eyes can't see.
Sources of Light
We can't see anything in a dark room, but when we turn on a light bulb or light
a match we can see the things in the room. The glowing bulb or the burning
match are sources of light.
A light bulb or a match is man made. Such a man made source of light is
called an artificial source of light. Burning wood, coal, oil and cooking gas
are some other examples of artificial source of light. The chemical compounds
in the sparkle burn and produce coloured light. When the current flows
through the thin filament of an electric bulb, it emits light. Light is emitted when
current passes through the vapour in tube lights, mercury and sodium vapour
There are other sources of light which are not man made. The sun and other
stars emit light too. They are called natural sources of light.
Objects such as books, tables, flowers, moon and planets scatter the light
falling on them. We see them by the light scattered from them. We cannot see
such objects in the dark. These are called non-luminous objects.
Propagation of light through media
• If we hold a piece of glass in front of a burning candle we can see the
candle but if we hold a piece of wood in front of it we can't see it. That
means light from the candle can travel through glass and reach our
eyes but the light cannot travel through the piece of wood.
• Light can travel through media such as glass, water and air. They are
called transparent objects. Light can also travel through vacuum.
• Light cannot pass through objects such as wood, stone and metal.
We call these opaque objects.
• Objects like wax or oilpaper transmit a part of the light falling upon
them. Such objects are called translucent objects.
Rectilinear propagation of light
• Light travels along a straight line. The velocity of light in air or vacuum
is 3,00,000 Km/s
• We can do a simple experiment to verify that light travels in a straight
line. Take two square hard boards of equal size and make pin holes at
their centres. Fix the two hard boards vertically with two stands. Place
one hard board in front of a burning candle. Peep through the pin hole
and make sure that you can see the flame. Place the second hard
board in between the candle and the first hard board. Now try to see
the flame through the first pin hole. If you can't, move the second hard
board till you can see the flame. Are the two pin holes and the flame in
a straight line, now?
• We see shadows everywhere. When we walk, our shadow walks with
us. On a hot summer day we sit under the shadow of a tree. You may
have noticed, in general shadow is formed when light falls on any
opaque object. This happens because light travels in a straight line.
Activity 1 – Pin hole camera
• Make a pinhole in the middle of one side of a hard board box. Cut off a
square on the opposite side and paste an oil paper over it. Point the
pin hole towards a distant object. Observe the inverted image formed
on the oil paper. Why is the image inverted?
How does a pinhole let you see the images carried by light and why are these images upside-down and backward?
The simplest way to understand pinhole images is to take a look at what happens to light rays reflecting from an
object,let's say a tree that you are looking at with your pinhole viewer. Sunlight bounces off the leaves, the trunk, the
ground beneath the tree. A pinhole lets through only a portion of these reflected light rays. Most of the light that reflects
from the tree is blocked by the cardboard surrounding the pinhole. Only a few of the rays of light that bounce off a
particular leaf are heading in just the right direction to shine through the pinhole of your viewer. Light rays from leaves
at the top of the tree must slope down at a steeper angle to pass through the pinhole. These rays hit the bottom of the
screen. Rays of light from the base of the tree trunk must slope up to pass through the pinhole. These upward moving
rays will hit the top of the screen. Think about this: if the rays from the top of the tree hit the bottom of the screen, and
the rays from the bottom of the tree hit the top of the screen, then seems like you should see an upside-down picture of
the tree on your screen. (And you do!) The same logic applies to right and left so that the image is also reversed this
way. The tree has more points than the top and bottom. The same rules apply to light rays reflecting from every point
on the tree's leaves and trunk: only the light rays that are heading in just the right direction will pass through the
pinhole. On the other side, these rays of light line up geometrically to make an upside-down and back
Activity –2 Playing with Shadows
Reflect the sunlight with a plane mirror and make it fall on a small hole in hard
board sheet. Let the light beam coming out by the hole fall on a white screen.
Place a small ball in the path of the light between the hole and the screen.
Observe the dark shadow of the ball on the screen. Draw the outline of the
shadow with a pencil. Observe what happens to the size of the image if the
screen is moved a little away from the ball. Explain the reason for this.
Make the hole on the hard board larger and repeat the experiment. Observe that
the shadow formed consists of a central dark region called umbra and a partially
dark region called penumbra around it. Mark the shadows on the screen. What
do you understand from this experiment?
Solar eclipse is
nothing but moon's
shadow on earth
Colors of Light
You might have seen the rainbow on a rainy day either in the morning or
evening on the side opposite to the sun. The seven colours violet, indigo, blue,
green, yellow, orange and red are seen from the bottom of the rainbow. White
light is dispersed into seven colours by the raindrops. This forms the rainbow.
Pass sunlight slanting through one side of an equilateral glass prism and
observe the colours of light on the other side of the prism. List the colours in
order. Isaac Newton observed the pattern of colours of white light and called it
Why do the objects have different colours? Of the seven colours of white light a
few colours are absorbed and certain other colours are reflected by the objects.
The colours so reflected are seen by us as the colours of the objects. For
example, when white light falls on a green leaf, green colour alone is reflected;
and the rest of the colours are absorbed. That is why the leaf appears green
Optical Instruments
Human eye is an amazing device with which we can see a lot of things. We
can see stars millions of miles away, we can pass a hair through a needle hole
just by looking carefully. But there is a limit to how much details we can see at
a distance, or how small an object is visible to us.
An ant has several thousand eyes. The moon has mountains and valleys and
big crators on its surface. We can't see those with bare eyes. Our eyes can not
see our face! Or a bicycle coming from the other corner of a street! Optical
instruments are clever devices to help us see things that we don't see with
bare eyes. For example, mirror is an optical instrument that we use every day.
• Mirrors can be of different types:
1. A plane mirror is a plane glass whose one side is coated with mercury.
2. A convex mirror is a spherical glass whose inner surface is coated with
mercury. The convex mirror is used as rear view mirror in vehicles.
3. A concave mirror is a spherical glass whose outer surface is coated with
mercury. The concave mirror is used in the headlight of vehicles to reflect
the light and render parallel beam.
fig: A boy standing in front of a large concave mirror. A
magnified virtual image is formed. When he moves back
the image inverts!)
Our eyes
The lens inside our eye is soft and can change shape. That is necessary for
focussing objects at various distances on the retina. When we are looking at a
distant object the lens stretches and becomes thin. As an object moves closer
and closer the lens becomes thicker and thicker to focus the object on retina.
If the lens can not become thick enough to focus nearby objects or the eyeball
flattens, they appear to be out of focus. Only distant objects are seen clearly.
This defect is known as long sightedness. To rectify this defect spectacles
having convex lenses are used.
If, on the other hand, the shape of the eyeball is elongated or the lens can not
become thin enough, we can see the nearby objects clearly; the distant
objects cannot be seen clearly. This is defect is known as short sightedness.
To rectify this defect spectacles having concave lenses are used
Other optical instruments
• Microscopes are used to magnify very tiny objects. A microscope is
essentially made up of two convex lenses. Telescopes are used to
see distant objects clearly. A telescope is made up of convex lenses
and possibly concave mirrors. Cameras, projectors, binoculars and
periscopes are examples of optical instruments.
Basic ideas learnt
Light is a form of energy.
Natural sources of light emit light of their own.
Non luminous objects are seen by the light scattered from them.
Objects can be classified into transparent, opaque and translucent
Light travels in a straight line.
The velocity of light in air or vacuum is 3,00,000 Km/s.
The shadow of objects are formed because of the rectilinear
propagation of light.
White colour is the composition of seven colours: violet, indigo, blue,
green, yellow, orange and red.
Short-sighted persons use spectacles with concave lenses.
Long-sighted persons use spectacles with convex lenses.
Microscopes are used to magnify very tiny objects.
Telescopes are used to see distant objects
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