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Each of these papers was of historic importance. In honour of this great year in the history of Physics, we are celebrating its centennial as “The International Year of Physics”.

Here at Indrandhanush we will be observing this year in a special way, with a journey of discovery.

Would you too like to become a great scientist, like Einstein, or Archimedes, like Aryabhata, Marie Curie or C.V.Raman ? How does one become a great scientist ?

You don't become a great scientist by mugging up books on General Knowledge for taking part in quiz competitions. You don't become a good scientist by learning lessons by heart and getting good marks in examinations. You don't become a scientist until you learn to understand things thoroughly. And understanding things deeply is only the first step. What is most important is learning to ask questions that nobody has asked earlier and keeping at it till you find a satisfactory answer

Asking new questions and finding out the answers for yourself is the only way to do good science. So in this series of articles, we are going to suggest some important experiments that you can do yourself. These experiments can be done with simple apparatus that you can make yourself. But the questions that these experiments give rise to may not be so simple . By answering these questions, you will train yourself to become a good scientist. Let us start our journey.

The Magic Mirrors experiment:

You can do this experiment with a single pocket mirror, or with a few different pocket mirrors. You can buy these for about 5-10 rupees each from a bazaar shop.

Lets assume that you have three small rectangular pocket mirrors for this experiment. (You can do this experiment with a single pocket mirror also if you dont have three mirrors). Now make three card rectangles which cover the mirrored glass exactly. In one card rectangle cut a hole which is circular in shape. The diameter of the circle should be around 3 cm. In the second card cut a hole in the shape of a square of side 3 cm. In the third card, the hole should be in the shape of an equilateral triangle of side three cm.

Paste these three masks on the three mirrors with some sticky adhesive, and after the gum has dried, carefully clean the exposed mirrors. So now we have three mirrors in the shape of a circle, square and triangle.

Now you are ready to do this intriguing experiment. Go out in the sunlight and using the circular mirrror cast a reflection of the sun onto the shirt of a friend who is standing one meter away from you. What is the shape of the light figure which forms on his shirt ?

Repeat the experiment with the square mirror and the triangle mirror. What is the shape of the light reflection on your friend's shirt ?

Now throw the reflection of the circular mirror on a distant white wall.(which is about 20 metres away) What is the shape of the reflection ?

Repeat the experiment with the square and the triangle mirror, throwing their reflection on the distant wall. What do you observe ? Are you surprised ? What is the reason for this puzzling experiment? Can you think of a good explanation ?

Next month, we give the explanation, and also continue our journey of discovery.



Remember the Magic Mirrors experiment I asked you to do last month. Did you perform the experiment ? If you did, then did you observe the following puzzling facts ?

  When the reflected image is taken up on a nearby screen, it is the same     shape as the mirror.

Like this : ( Here put figure of a circle, a square and a triangle )

  When you take the image on a wall far away, all the images are round in     shape , like this

( here draw three circles).

Since you are all good scientists I am sure that you must have slowly increased the distance between the screen and the mirrors and observed how the triangle and square gradually becomes a circular disc.

What is this circular disc ?

What happens if we use a smaller mirror , i.e. if we reduce the size of the hole in the mask covering the mirror ?

Why does a smaller mirror not give us a smaller image at a long distance ?

Of course, since you are all good scientists, you will instantly exclaim “ Don't tell me the answer”- because you will want to discover the answers for yourself. This is the real quality of a good scientist.

O.K. ! So you tell me why the image is circular at a long distance.

“Because the mirror then behaves like a pinhole and we have a pinhole projector which projects the image of the sun”, you tell me- and indeed, you are quite right. That is why the image is a circular disc- because the sun itself is round .

By increasing the distance between the screen and the mirror you can get a bigger and bigger image of the sun. In fact, you will discover that the diameter of the image is always roughly one hundred times smaller than the distance between mirror and screen. At 100 metres, we will get a sun image which is almost one meter across. So we can use this idea to construct a powerful solar telescope and see what is happening on the surface of the sun. We can use it to see sunspots and show them to our parents and friends.

The photograph of sunspots on page ...... has been obtained by using just one such solar telescope which I made last year to observe the transit of Venus. It was taken during a lecture at Ferguson college Pune. You too can get a wonderful and exciting image of the sun, as big as you wish.

“Can I get an image of the sun which is ten metres across by increasing the distance to one kilometre”?, you will ask me. Yes, indeed you can, but it is not as easy as it sounds. For the greater the distance, the dimmer the image.

To see it properly you will have to create a room which is as dark as possible in which to take the image. The second thing you need is a very stable mount for your small mirror, which you can adjust carefully. Any telescope is pretty useless unless you have a good and steady mount. Otherwise, the smallest motion of the mirror will make you lose the image.

You can make a beautiful mount by buying a large plastic ball and filling it with sand. A large spherical plastic lota will also do. Please don't ask me for details about how to make a room dark, how to mount the mirror on the ball, or how to place the plastic ball in a ring, or where to get the ring, and such questions. No good scientist asks for answers to such questions. On the contrary, I expect to hear from you about how you solved the problems and did the experiment, and even a photograph or diagram of the sunspots you saw.

One thing I will share with you : To see sunspots you will have to get as sharp an image as possible. You can get a sharp image by reducing the size of the mirror, but not too much. How much ? That you have to decide by trial and error. I got good results with a 2 cm diameter for my mirror and projecting over a distance of around 30 metres into a dark room.

If you and your friends successfully do the above experiment and see sunspots, please write to us with all the details of your experiment. You will be rewarded with a prize – a copy of my book “Measuring the Universe with a String and Stone”.

Next month we will discuss what we can discover about sunspots, the sun and our earth with our powerful solar telescope.


Last month we saw how to make a powerful solar telescope using a simple pocket mirror, a dark room, and a large plastic ball filled with mud or sand.

I had asked you to do an experiment with this solar telescope to get a large image of the sun on which you might see sunspots. I also asked you to write back to Indradhanush describing what you did and what you saw. In this article, I was planning to tell you what you can learn by observing sunspots. But there is only one problem. So far nobody has written back to tell us about their experiment. Nobody has told us about whether they have succeeded or failed in getting the sun's image and seeing sunspots. So if none of my readers has seen sunspots, then what is the point in writing about what you can discover using sunspots ?

For this reason, I have decided to write about some other simple experiments that you can do with your big plastic ball filled with sand. I will write about sunspots only when I get at least one letter telling me about your experiment with the ball and mirror solar telescope. In the meanwhile, here are two more simple but interesting experiments that everyone can do.

Experiment 1.Take a large plastic ball, which is easily available in any toy shop. Buy a straight 30 cm long piece of aluminium or plastic tube from a hardware store. If you cannot obtain a metal or plastic tube, you can use a suitable substitute like a hollow bamboo tube about one foot in length. (The tube should be narrow and straight and you should be able to look through it to see the stars in the sky at night.). Using a hot knife cut small holes at the two opposite ends of the plastic ball . Put the hollow tube through the ball, such that the two ends of the tube stick out on both sides of the ball. Fill the ball with sand and seal the openings with sticky tape. Use a cup or a ring as the base mount for the ball in the following experiment.

At night, go out into the open and place your instrument on a high stool on its ring mount. Using it like a telescope , or like a rifle barrel, look through the tube and take aim on any star. Fix the ball on its mount such that you can see that star when you look through the tube. After fifteen minutes, without moving the ball again look through the tube. Now you cannot see the star. Why ?

Experiment 2. Look towards the north and identify the pole star (Dhruva tara) in the sky. Point your instrument so that it is aimed at Dhruva tara, and you can see dhruva tara when you look through the tube. After 15 minutes again look through the tube without moving the ball on its mount. You can still see dhruva tara .

Look through the tube after one hour, after two hours, after 25 hours, after 49 hours, after three days or three weeks or three months …without moving your ball on its mount – you will always see Dhruva tara.

This is because Dhruva tara does not move in the sky at all. (In fact it moves just a little bit, which you can hardly see, but you might be able to observe this movement with a very thin tube).

All the stars and planets move in the sky except Dhruva tara. Why ?

The next thing you need to do is to measure the angle made by Dhruva tara above the horizon. Since Dhruva tara does not move, that angle is fixed. It does not change. This angle is just the latitude of the place where you are. If you are in Simla, or Mandi, you can measure your latitude by measuring the angle of Dhruva tara above the northern horizon. How do you measure this angle ? This we will discuss in the next article, where we will also learn to make a geosynchron.


How to Make a Geosynchron.

We learn in school that the earth rotates around its axis once in a day , and that it also revolves around the sun once in a year. A geosynchron model helps us to understand this.

In my last article, Discover it 3, I promised to tell you how to make a geosynchron. Take a large spherical globe and place a tube through it as was described in my last article for doing the Dhruva tara experiment. You can use the plastic globe models of the earth which are available in most schools for this experiment, or else, you can use the same plastic ball which you can purchase in most toy shops. The metal tube through which you are going to look should pass through the centre of the sphere and come out at both ends at the north pole and at the south pole of the globe. (When you do this the metal tube will now be in the same position as the axis of the globe model of the earth)

As in the last experiment, set the globe on its ring mount outside, and fix it such that you can see Dhruva tara through the tube.

Without changing the position of the metal tube rotate the globe so that the map of India is on top. When you have completed this whole procedure, you have made a geosynchron.

What is a geosynchron (GS) ? It is a model of the earth which is exactly parallel to our real earth. India on our GS is parallel to real India . Australia on our GS will also be parallel to Australia in its real position in the southern hemisphere.

When it is noon in India , the sun will be right above our GS and it will be noon on our GS. When the sun is setting in real India , you will see that it is also setting for the small India map which is on the GS. In fact whatever time the sun is showing in real India , the same solar time will be seen on the India which is on our GS. Geosynchron means ‘same earth time'. It could also be called a Samantar Prithvi.

By the simple act of fixing the line of sight of the tube onto Dhruva tara, we have created a parallel earth which remains parallel to the real earth not only as the earth rotates around its own axis, but also as the earth goes around the sun in one year.

At noon, the shadow cast by an upright pole is shortest. As the earth rotates around the sun, the length of the shadow cast by an upright pole by the noonday sun changes from day to day and month to month. From the length of the noon day shadow, we could , if we wanted, construct a solar calendar. The noonday sun shadow of an upright pole would be longer in winter and shorter in summer. At the tropics it is zero on June 21 st - the day of the summer solstice.

Since our geosynchron is exactly parallel to the real earth, we could place an upright pin on the map of India , and find out that the pin casts the same kind of shadow as an upright pole on the open ground. So we could construct a small sundial and place it on the GS India , which would tell the same time as a real full size sundial on real India .

Why does Dhruva tara always remain fixed ? This is because the earth's axis, like the axis of a spinning top, always points in a fixed direction, and Dhruva tara happens to be in that direction. Secondly, because Dhruva tara is very very far away, so that even if the earth moves round the sun, that movement is very small compared to the distance of the sun and earth from Dhruva tara.

Dhruva tara is not the only celestial object which appears to remain fixed in the sky. There are some man made satellites which also appear to be fixed in the sky. These are the geosynchronous satellites on which are fixed the antennas, receivers and transmitters which are used for mobile phone and internet telecommunications. These satellites are all at a height which is tens of thousands of kilometres above the earth's surface. At this height, the period of rotation of the satellite around the earth is equal to the period of rotation of the earth and therefore the satellite appears to be fixed above the earth.

Is it not wonderful that for two completely different reasons two kinds of celestial objects appear to be fixed in the sky ? Dhruva tara, because it is extremely far away and because our earth's axis points at it . Geosynchronous satellites, because they are just so far away that they revolve around the earth at the correct speed.

Supposing you were sitting on a geosynchronous satellite which is just above India and looking down on real India from outer space. You would see the sun rising and setting on India . You would see the line of light and shadow which separates day from night slowly rotating around the globe once in 24 hours. You can have the same experience by standing next to your geosynchron on an open ground and looking at it at different times of the day.

You can imagine, that along that line which separates light from dark, millions of people are waking up, going to the bathroom, brushing their teeth, and getting ready to go to school or to work. Along another line on the other side of the globe, millions more are relaxing at home after a hard days work and watching TV, and getting ready for dinner. Enjoy your journey to the geosynchronous satellite thousands of kilometres above the earth in outer space.


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