An opaque object blocking the path of light is called a shadow. A shadow is a dark region, and is formed only when a light source, an opaque object and a screen are present.
Opaque objects form shadows because light is not able to bend around them.Shadows are seen more clearly on light screens. Whatever the colour of the object, its shadow is always black because it is not illuminated by light. The length and shape of a shadow depends on the object by which it is formed.
Now, one by one hold each of the opaque objects in the sunlight, slightly above the ground. What do you see on the ground? You know that the dark patch formed by each on the ground is due to its shadow. Sometimes you can identify the object by looking at its shadow (Fig. 1). Spread a sheet of paper on the ground. Hold a familiar opaque object at some height, so that its shadow is formed on the sheet of paper on the ground. Ask one of your friends to draw
Fig. 1. Sometimes shadow of an object gives an idea about its shape
the outline of the shadow while you are holding the object. Draw outlines of the shadows of other objects in a similar way.
Now, ask some other friends to identify the objects from these outlines of shadows. How many objects are they able to identify correctly?
Do you observe your shadow in a dark room or at night when there is no light? Do you observe a shadow when there is just a source of light and nothing else, in a room? It seems we need a source of light and an opaque object, to see a shadow. Is there anything else required?
This is an activity that you will have to do in the dark. In the evening, go out in an open ground with a few friends. Take a torch and a large sheet of cardboard with you. Hold the torch close to the ground and shine it upwards so that its light falls on your friend's face. You now have a source of light that is falling on an opaque object. If there were no trees, building or any other object behind your friend, would you see the shadow of your friend's head? This does not mean
Fig. 2. A shadow is obtained only on a screen
that there is no shadow. After all, the light from the torch is not able to pass through his body to the other side.Now, ask another friend to hold the cardboard sheet behind your friend. Is the shadow now seen on the cardboard sheet ? Thus, the shadow can be seen only on a screen. The ground, walls of a room,
Fig 3. Shadows of animals hidden in your hand
a building, or other such surfaces act as a screen for the shadows you observe in everyday life. Shadows give us some information about shapes of objects. Sometimes, shadows can also mislead us about the shape of the object. In Fig. 3. are a few shadows that we can create with our hands and make-believe that they are shadows of different animals. Have fun!
Place a chair in the school ground on a sunny day. What do you observe from the shadow of the chair?
Does the shadow give an accurate picture of the shape of the chair? If the chair is turned around a little, how does the shape of the shadow change?
Take a thin notebook and look at its shadow. Then, take a rectangular box and look at its shadow. Do the two shadows seem to have a similar shape?
Take flowers or other objects of different colours and look at their shadows. A red rose and a yellow rose, for instance. Do the shadows look different in colour, when the colours of the objects are different?
Take a long box and look at its shadow on the ground. When you move the box around, you may see that the size of the shadow changes. When is the shadow of the box the shortest, when the long side of the box is pointed towards the Sun or when the short side is pointing towards the Sun?
Let us use this long box, to prepare a simple camera.
The image formed by a pinhole camera is inverted and smaller in size when compared to the original object. These cameras work on the principle that light travels in a straight line. Pinhole cameras are cheap and simple to make. An eclipse can be viewed using a pinhole camera.
Take two boxes so that one can slide into another with no gap in between them. Cut open one side of each box. On the opposite face of the larger box, make a small hole in the middle [Fig. 4 (a).]. In the smaller box, cut out from the middle a square with a side of about 5 to 6 cm. Cover this open square in the box with tracing paper (translucent screen) [Fig. 4 (b).]. Slide the smaller box inside the larger one with the hole, in such a way that the side with the tracing paper is inside [Fig. 4 (c).]. Your pin hole camera is ready for use. Holding the pin hole camera look through the open face of the smaller box. You should use a piece of black cloth to cover your head and the pinhole camera. Now, try to look at some distant objects like a tree or a building through the pinhole camera. Make sure that the objects you wish to look at through your
Fig. 11.4 . A sliding pin hole camera
pinhole camera are in bright sun shine. Move the smaller box forward or backward till you get a picture on the tracing paper pasted at the other end. Are these pin hole images different from their shadows?
Look through your pin hole camera at the vehicles and people moving on the road in bright sun light. Do the pictures seen in the camera show the colours of the objects on the other side? Are the images erect or upside down? Surprise, surprise!
Let us now image the Sun, with our pin hole camera. We need a slightly different set up for this. We just need a large sheet of cardboard with a small pin hole in the middle. Hold the sheet up in the Sun and let its shadow fall on a clear area. Do you see a small circular image of the Sun in the middle of the shadow of the cardboard sheet?
Look at these pin hole images of the Sun when an eclipse is visible from your location. Adjust your pin hole and screen to get a clear image before the eclipse is to occur. Look at the image as the eclipse begins. You will notice a part of the Sun's image gradually becoming darker as the eclipse starts. Never ever look directly at the Sun. That could be extremely harmful for the eyes. There is an interesting pin hole camera in Nature. Sometimes, when we pass under a tree covered with large number of leaves, we notice small patches of sun light under it (Fig. 11.5). These circular images are, in fact, pin hole images of the Sun. The gaps between the leaves, act as the pin holes. These gaps are all kinds of irregular shapes, but, we can see circular images of the Sun. Try to locate images of the
Fig. 11.5. A natural pinhole camera. Pinhole images of the Sun under a tree!
Sun when an eclipse occurs next. That could be so much fun! Boojho has this thought. We saw upside down images of people on the road, with our pinhole camera. What about the images of the Sun? Did we notice them to be upside down or anything like that?
Paheli has another thought. Surely, all these results that we are seeing, formation of shadows and pinhole images are possible only if light moves in a straight path?
Let us use a piece of a pipe or a long rubber tube. Light a candle and fix it on a table at one end of the room. Now standing at the other end of the room look at the candle through the pipe [Fig. 11.6.(a)].
Fig. 11.6. Looking through a pipe pointed
(A) towards and (B) a little away from a candle
Is the candle visible? Bend the pipe a little while you are looking at the candle [Fig. 11.6 (b)]. Is the candle visible now? Turn the pipe a little to your right or left. Can you see the candle now?
What do you conclude from this?
This suggests that light travels along a straight line, isn’t it? That is why, when opaque objects obstruct it, a shadow forms.
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