Light travels in straight lines and can only be seen when it reflects off objects into our eyes. Light comes from sources like the sun, lamps, or candles. When light travels through or reflects off objects, shadows are formed behind the objects where light is blocked. Light is made of different colors that are visible when separated using prisms or rainbows. Plants need light to grow properly and different colored lights can affect plant growth.

1. PROPAGATION OF
LIGHT

2. • We see many things around us. What
makes it possible for us to see them? At
night, when we switch on the light. We
can see the books on the table, the
clock on the wall, the clothes on the
stand. But we cannot see anything if the
lights are off. We say that light makes
things visible to us.

3. • Can we see the
entire book when
the light from a
table lamp falls on
it?
• No.
• We only see those
parts on which the
light falls and gets
reflected into our
eyes.

4. 4
• When travelling in a bus you must have seen its
windscreen, the big glass window in front of the driver.
It is always clean. But, do you notice it easily ? No, you
don't. It is because light rays pass right through it. We
easily see a vehicle in front of the bus through the
windscreen but not the windscreen itself.

5. 5
• Look carefully at a
beam of sunlight
entering through the
roof. You see a spot of
sunlight on the floor.
• In the beam, you see
dust particles moving
randomly.
• In Fact, it is because of
these dust particles
that we see the path
of light.
• What is this path like ?
It is always straight.
• This is called the
propagation of light

6. 6
• LASER BEAM VIDEO

7. • Nowadays, we get laser torches that give red
light. This torch gives out a fine beam of light
which travels very far. Shine this beam on a
distant wall and obtain a spot of red light on
the wall. You will easily see that the path of
light is a straight line.

8. • Take a half-metre length of a plastic tube with a diameter
of 1cm. Look through it at some object. Now bend the
tube a little. You will not be able to see that object now.
At first, when the tube was straight, the light rays coming
from the object could travel through it and enter your eye.
However, now that the tube is bent, that is not possible.
This tells us that light travels in a straight line.

9. • Take three square pieces of cardboard as shown in the
figure.
• Make a fine hole exactly in the centre of each of the
pieces. With the help of stands place these squares one
behind the other at intervals of 1 m.
• Take a long stiff wire and ensure that it passes through all
three holes.
• Place a lighted candle at one end in front of the hole. Now
look through the hole in the cardboard at the other end.
You will easily see the light from the candle.
• Now move any one of the three squares slightly. Do you
see the light now ? You do not. It is because the three
holes are not in a straight line any more.
• This shows that light always travels in a straight line.
When light travels through a circular hole you get a
circular spot of light on a wall on the other side.

10. 10
When light travels through a circular hole you
get a circular spot of light on a wall on the
other side.
What can you infer from this?

11. 11
• Light travels in a straight line.
• This is called the linear
propagation of light

12. What makes it possible to see the path of light
in the darkness of a cinema hall?

13. 01/09/16
Formation of shadows
• You might have played at casting shadows of
the shape of a dog or rabbit or bird, in the
light of a lantern, with the help of your hands
and Fingers.
• How does an object cast a shadow?
• The shadow cast by a point source of light

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• Take a candle, bulb or a torch as a source of light.
• Place a piece of cardboard with a hole in its centre, in front of
this source as shown in the figure. Now the light can be seen
emerging from the fine hole O. Such a source of light is called a
point source.
• At a distance of about lm in front of the cardboard, place an
upright screen.
• Between the source O and the screen, suspend a hall PQ. Now
observe the shadow AB. of the ball PQ.
• The rays OP and OQ from the point source pass the ball just
touching it at P and Q and fall on the screen at points A and B
respectively. However, no rays reach the part of the screen
between A and B. Hence, this part remains dark. This is the dark
shadow called the umbra.

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• What happens if the screen with the hole is removed? We
do not have a point source any more. This is called an
extended source. What is the nature of the shadow
caused by an extended source ?
• In the figure alongside, MN is an extended source. PQ
• is the ball placed in front of it. As a result of the extended
source we see a shadow which has two parts, a faint part
and a darker part.

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• Keeping the distance between the extended source and
the ball the same, move the screen further away from
the ball. Observe the shadow of the ball. As the screen
moves further and further away the umbra becomes
smaller and smaller till finally it completely disappears.
This shows why an aeroplane or a bird does not cast a
shadow when they are very high above the earth.

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• We saw how, when there is a source of
light, an object casts a shadow on a screen.
• The sun is a large source of light in the sky.
The planet earth and its satellite, the moon,
are also in the sky. Why do we not see their
shadows ?
• Because there is no screen on which these
shadows can fall.
• But when the sun, moon and earth happen
to come in a straight line, we say that an
eclipse has occurred.

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http://spaceplace.nasa.gov/review/venus-transit/solar-eclipse-cartoon-lrg.en.jpg

19. • If the moon comes between the sun and the earth
and the three are in a straight line, the shadow of
the moon falls on the earth and we say that a solar
eclipse has taken place.
• A solar eclipse takes place only on a new moon
day.
• The solar eclipse may be total or partial.
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20. • If the earth comes in between the sun and the
moon, and the three are in a straight line, the
shadow of the earth falls on the moon and a lunar
eclipse takes place.
• Naturally, a lunar eclipse takes place only on a full
moon day. It may be either total or partial
01/09/16
http://mars.nasa.gov/images/Lunar_eclipse_sideview.jpg

21. THE COLOURS IN SUNLIGHT
• We generally see sunlight as being
yellowish white.
• However, we are not aware that there
are many colours mixed in it.01/09/16

22. • While painting a house, the painter uses a
white base to get the desired shade.
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23. • Cut out a circular piece of cardboard.
• Make a hole in its centre and pass the
spoke of a bicycle wheel or a long nail
through it in such a way that this disc
will be able to tum around it.
• Draw seven lines from the centre to the
periphery at equal intervals, so that we
get seven equal triangles on the circle.
• Colour the triangles violet, indigo, blue,
green, yellow, orange and red
respectively.
• Now, holding the nail in your hand turn
the disc.
• While the motion of the disc is slow, we
can see the different colours distinctly.
But when the turning disc reaches a
certain speed it appears to be white.
This tells us that when seven colours
are mixed together we get white.

24. • Take a glass prism. It has two triangular sides.
• Take a cardboard and make a fine hole in it. Do
this experiment in a room where you get a
beam of sunlight from a window. Close the
window in such a way that you get a single
sunbeam into the room.
• Place the prism on a table with its base facing
you. Make sure the room is dark enough.
• Through the hole in the cardboard, allow the
sunbeam to fall on the prism at a slant. Now
look from the other side. You will see a band of
seven colours called the spectrum.
• If you allow the spectrum to fall on a white
screen you will see the seven colours distinctly.
You will see the red colour at the top of the
spectrum and the violet at the bottom. In
between, you will see orange, yellow, green,
blue and indigo in that order. This tells us that
sunlight can be broken up into seven colours.
01/09/16

25. • Have you seen these seven colours of sunlight
anywhere else ?
• Sometimes, after a shower, if you are standing
with your back to the sun, you may see the arch of
the rainbow in the sky in front of you. In the
rainbow, too, you will see the red colour at the top
and violet at the bottom. We ’ see a rainbow when
the sun's rays pass through the raindrops in the
sky. It means that the raindrops, like the prism,
break up sunlight into its seven, colours.01/09/16

26. The pinhole camera
• You have seen that light travels in a straight line. Using this
property you can easily make a special camera of your own.
• Take a square or round cardboard box. The box used for keeping
shuttlecocks would be ideal.
• Remove one lid and in its place cover the mouth with a piece of
white kite paper. Make a fine hole in the centre of the other lid of
the box. Light a candle and place it so that its flame will be directly
in front of the hole in the base. Now you will get an upside down
image of the flame on the kite paper screen. It is clear from the
diagram that the image of the flame is inverted because of the
linear propagation of light through the hole.
01/09/16

27. • Take some soil in two small dishes. Plant about 25 grains of wheat in each dish. Place
one dish in complete darkness. Place the other where it will get plenty of light. Water
them a little every day. After a week observe the two dishes and record your
observations as shown in the following table
• This tells us that light is necessary for the growth of plants. What is the effect of
different colours of light on the growth of plants ?
Point of observation Dish in darkness Dish receiving light
Height of the plants
Colour of the plants
Condition of the plant
Short
Yellowish green
weak
Tall
Deep green
strong

28. • Take red and blue Cellophane papers. Take some soil in three
beakers and plant some mustard seeds in each of them. Water the
three beakers and wrap two of the beakers completely in the red
and blue paper each.
• Do not wrap any paper around the third one. Place all three
beakers in sunlight. Observe them after a week. You will see
differences in the growth and height of the plants.
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29. • In the beaker that was not wrapped, most seeds are seen to have
grown well. In the beaker wrapped in red paper you will see that
only a few seeds have started growing but those which have grown
are tall. In the beaker wrapped in blue paper many more seeds will
be seen to have started growing but these seedlings are somewhat
shorter. This tells us that different colours of light have different
effects on the growth of plants.
01/09/16

30. • Production of food
• Plants need light mainly for producing food.
The green colour of a plant’s leaves is due to
the green substance that they contain. It is
called chlorophyll. Chlorophyll helps to
convert light energy into chemical energy.
This chemical energy is stored in plants in the
form of food.
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