The smoke chamber
R. Shankar, The Institute of Mathematical Sciences, Chennai
Vikram, an elephant in the Mudumalai wild life sanctury, and his friend Lakshmi, a mahout's daughter, were relaxing under a tree beside the river on a lazy sunday morning.
"Yesterday, one tourist was showing his children to make fire using a piece of glass", said Vikram. "He kept the glass above some dry leaves and after some time the leaves caught fire.  Later, I did the same thing but nothing happened".
"Ah! the tourist must have been using a lens" said Lakshmi, "It is not an ordinary piece of glass. It has a special shape. Come I'll show you, we have one at home and my mother once showed me this trick." She got up, climbed on to Vikram's back and they went back to the elephant camp.
When they reached Lakshmi's home she went in and returned holding a round glass with handle. "My father was very perturbed about your story," she said. "The part about starting a fire". "Yes," said Vikram.  "I should have told you: that man told his children to be very sure that the fire is completely put out, especially if there were dry leaves and things nearby." 
"Good," said Lakshmi. "Well, this is the lens. It is also called a magnifying glass because when you look through it, things look bigger".
"Oh! is that so ? Let me see". Vikram took the magnifying glass by the handle and peered through it. "I can't see anything, it is all blurred".
"You have to adjust the distance correctly. I think you will find it pretty difficult though, these were not made for elephants.  Doesn't matter, let me show you how to make fire using the lens".
She put a few pieces of paper on the ground and held the lens above them.  She adjusted the distance so that there was a very bright spot on the paper.  She held the lens steady in that position for a minute or two. Then smoke started coming out of the spot and soon the paper was in flames.
"Awesome! That magnificient glass seems to put all the light to one point!" exclaimed Vikram, "The places around the point that caught fire were quite dark whereas that point was very bright".
Lakshmi smiled, "Magnifying glass not magnificient glass, but yes you are right. The region around the point was like a shadow region. Somehow the lens collected all the light at that point".
Both of them thought about it for some time. They did it again and again observing carefully.
"Maybe the lens makes the light bend towards the centre", mused Vikram. "If it did that, then it would all meet at a point like we are seeing".
"Good idea!" responded Lakshmi. "Then the light rays will form a cone after passing through the lens and the tip of the cone will be the bright spot. How do we check this out?" It took only a minute for them to realise that "we could find out where the cone is by putting a stick in the middle and seeing where it blocks the light".
So they did the experiment again. Vikram held the lens steady with the height adjusted so that the bright spot was on a piece of flat stone on the ground. Lakshmi slowly pushed a pencil under the lens and they both looked to see when it affected the bright spot. They found that when the pencil was near the ground, the spot was unaffected until the pencil reached almost the center. On the other hand when it was higher and closer to the lens, its effects could be seen when it was at some distance from the center.
"It does seem as if the light rays are forming a cone" said Vikram after they had experimented with the pencil for some time. "But it would nice if we could see it more directly. If there were some way of making the rays visible, then we could directly see if they bend and form a cone".  
"Oh! The smoke chamber!!" yelled Lakshmi. "A friend of mine in school once told me he had seen it in a science exhibition. I wanted to try it out but never got around to doing it! This is the time, come let's make one".
"Slowly! Slowly!" said Vikram calmly. "I don't even know what you are talking about".
So she explained and they made one. They took a big cardboard box with one side open. They stuck cellophane paper on the open side so they could see through it. On the opposite side they made many small holes. In some of the holes, they inserted oodhuvathi's (agarbathis/incense sticks) and the other holes were for ventilation. They made a hole in one of the side walls and fitted a torch there.
When they put the box in a darkened room, lit the agarbathis so that they produced smoke and put on the torch, they could clearly see the beam. Then they did many experiments putting mirrors, the lens, bottles of water of different shapes in the way of the beam and learnt a lot by observing what each of these objects did to the beam.
Why don't you also do it and find out?