DISCOVER IT – 2

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 ?

1.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 )

2.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.