PROPAGATION OF LIGHT

• 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.

• 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.

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• 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.

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• 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

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• LASER BEAM VIDEO

• 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.

• 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.

• 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.

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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?

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• Light travels in a straight line. • This is called the linear

propagation of light

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

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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.

05/02/23http://spaceplace.nasa.gov/review/venus-transit/solar-eclipse-cartoon-lrg.en.jpg

• 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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• 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

05/02/23http://mars.nasa.gov/images/Lunar_eclipse_sideview.jpg

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. 05/02/23

• While painting a house, the painter uses a white base to get the desired shade.

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• 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.

• 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.

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• 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. 05/02/23

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.

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• 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 lightHeight of the plantsColour of the plants

Condition of the plant

ShortYellowish green

weak

TallDeep green

strong

• 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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• 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.

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• 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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