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Advanced Physics Topics like quantum mechanics and relativity have revolutionized our understanding of the universe.
How Images formed by a Pinhole Camera and Uses of Pinhole Camera
The pinhole camera consists of a closed box having a small pinhole in the front and a translucent screen at the back (see Figure). The translucent screen at the back side of the pinhole camera box is made of butter paper (also called tracing paper).
The butter paper acts as a screen to receive the image of the object. Since butter paper is translucent, some light can pass through it due to which we can see the image formed on it by keeping our eye behind the pinhole camera.
The pinhole camera is usually used to view the images of various objects like trees and buildings, etc. The pinhole camera can also be used to take photograph of an object if a photographic film is placed on the screen. The pinhole camera works on the principle that light travels in straight lines.
In other words, pinhole camera uses the fact that the light rays coming from an object and going in through the pinhole, travel in straight lines to the screen.
The pinhole camera shown in Figure has been pointed at a distant tree (which is the object here). If we look into the pinhole camera by keeping our eye at its back side, we will see an image (or picture) of the tree on the screen. This is because some of the light coming from the tree (or rather reflected by the tree) passes through the pinhole to form an image on the screen.
The formation of image in a pinhole camera can be explained as follows: A ray of light coming from the top point A of the tree passes through the pinhole in a straight line to form an image at point A’ on the screen (see Figure).
Another ray of light coming from the bottom B of the tree also passes through the pinhole in a straight line to form an image at point B’ on the screen. In the same way, each point on the tree AB forms its corresponding image on the screen between points A’ and B’. In this way, the image A’B’ of the tree AB is produced on the screen.
The light coming from tree passes through the pinhole of the pinhole camera in straight lines as shown by the straight lines AA’ and BB’ which represent rays of light and form the image in Figure. So, the working of a pinhole camera illustrates the property that light travels in straight lines.
Please note that the light rays AA’ and BB’ coming from the top and bottom of the tree respectively, cross-over (or intersect) at the pinhole P. Due to the crossing-over of light rays at the pinhole, the top of tree comes at the bottom in image, and the bottom of tree comes at the top in image. It appears as if the tree has been turned upside-down in the image. The upside-down image of an object formed by a pinhole camera is called an inverted image.
An inverted image is formed in a pinhole camera because the light rays coming from the top and bottom of the object cross-over (or intersect) at the pinhole. The image formed in a pinhole camera is real because it can be received on screen. Thus, a pinhole camera forms a real and inverted image of an object. The size of image formed by a pinhole camera depends on the distance of the object from the camera.
The size of image formed by a pinhole camera can be smaller than the object, equal to the object or bigger than the object depending on the distance of the object from the pinhole camera. Farther the object from the pinhole camera, smaller will be its image.
Another point to be noted is that the image formed in a pinhole camera is of the same colour as the object. For example, in Figure 10, the image of tree seen on the screen has the same colours as the tree itself.
The picture which we see on the screen of a pinhole camera is called image. The image in a pinhole camera has the following characteristics :
- The image in a pinhole camera is inverted (upside down) as compared to the object.
- The image in a pinhole camera is real (because it can be formed on a screen).
- The image in a pinhole camera is of the same colour as the object.
- The image in a pinhole camera can be smaller than the object, equal to the object or bigger than the object (depending on the distance of object from the pinhole camera).
In Figure we have shown the image of a tree formed by a pinhole camera. We can also view the images of other objects like a building, an electric bulb or a lighted candle, etc., through a pinhole camera.
The pinhole camera is a simple device to see the images of various objects. A pinhole camera can also be used to take the photograph of an object if a photographic film is placed on the screen. The pinhole cameras were used for taking photographs in earlier days. They are not used much now. These days we use lens cameras for taking photographs.
Activity To Make A Pinhole Camera
We can make a simple pinhole camera for viewing the images of various objects by using cardboard shoe boxes and tracing paper (or butter paper) as follows : Take a closed cardboard box A (say, a cardboard shoe box) (see Figure). Make a small hole in the middle of front side of the cardboard box A with a pin.
Cut out a square hole of about 5 cm in the back side of the cardboard box A facing the pinhole. Cover this open square hole with tracing paper (see Figure). This tracing paper will act as a translucent screen to receive the image. Take another similar cardboard box B which is slightly bigger than the first box A .
Cut open the front and back walls of this box. Now slide this hollow box B over the pinhole camera box A at the tracing paper end so that the tracing paper part is fully inside the bigger box. The purpose of enclosing the tracing paper part of pinhole camera in a bigger box is to make the surroundings of the tracing paper (translucent screen) dark so that the image formed on it can be seen clearly even during day light. Our pinhole camera is now ready for use.
Hold the pinhole camera in your hands so that the pinhole is towards the object (say, a distant tree or a building) which we want to observe. Keep your eye behind the open end of box B and look at the distant object through it. Move the bigger box forward or backward till you get an image (or picture) of the object on the tracing paper screen inside the pinhole camera. We can also observe the vehicles and people moving on the road in bright sunshine with this pinhole camera.
Please note that the pinhole is kept small so that only a very small number of light rays (from a given point of the object) pass through it and a sharp image of the object is formed on the screen. If, however, the pinhole is made bigger, then a large number of light rays from a given point of the object pass through it due to which a blurred image of the object is formed.
A Natural Pinhole Camera
The pinhole camera effect can be observed in everyday life. On a sunny day, when we pass under a tree covered with a very large number of leaves, we often see bright circular patches of light on the ground (under the tree). These bright circular patches of light are the pinhole images of the sun.
This is because the small holes between the clusters of leaves act as pinholes and light coming from the sun passes through these natural pinholes to form bright circular images of the sun on the ground below the shady tree. In this case, the sun is the object, the tiny gaps between leaves are the pinholes and the ground acts as the screen.
Shadows
When an object is placed in front of a source of light, it produces a shade (dark area) behind it. The ‘shade’ cast by an object is called its ‘shadow’. Shadows are formed when light is stopped by an object. An opaque object stops the light completely, so an opaque object casts a dark shadow behind it.
A translucent object stops the light partially, so a translucent object casts a weak shadow (less dark shadow). A transparent object does not stop any light from passing through it, so a transparent object does not cast any shadow behind it.
A shadow is formed when an opaque object comes in the path of light and stops it. An object forms shadow on the opposite side to the source of light. The shadows of objects are usually similar in outline to the objects and hence we can identify the objects from their shadows (see Figure).
We can see our shadow formed by an electric bulb fitted on one wall of a room. Our shadow will be formed on the opposite side to that of electric bulb (which is the source of light). Similarly, the shadow of a ceiling fan hung in the centre of a room will fall on the side opposite to the lighted electric bulb. A lighted candle fixed in a room will also cast our shadow and that of the ceiling fan on the opposite side.
The sunlight also forms shadows of the objects which are on the ground or near the ground. For example, if we stand in the sun, our body casts a shadow on the ground (on the side opposite to the sun). And when we walk in the sun, our shadow always walks with us.
Our shadow is very long in the morning when the sun just rises. The length of our shadow goes on decreasing till noon. Our shadow is shortest at noon. The length of our shadow starts increasing in the afternoon. And our shadow becomes very long in the evening, just before sun-set.
Please note that the shadow of an object can be seen only on a screen. The surfaces such as the wall of a room, a building and even ground act as screen for the shadows which we see in our everyday life. While performing activities on shadows, we can use a sheet of white cardboard as screen.
From this discussion we conclude that we require three things to observe a shadow :
- a source of light
- an opaque object (to obstruct the path of light), and
- a screen on which the shadow can be seen.
Please note that shadows are formed because light rays travel in straight lines, and they cannot bend round the corners of the objects. The shape of the shadow is also the same as the shape of the object because light travels in a straight line path. We will now describe an activity to show the formation of a shadow.
Activity For The Formation of A Shadow
We can show the formation of a shadow as follows : Take a torch and light it in a dark room. This torch is the source of light. A sheet of white cardboard is taken and held vertically in front of the torch at some distance from it (see Figure). This sheet of white cardboard will act as a screen (on which the shadow will be formed). Hold a key (or any other object) in front of the torch, close to the cardboard screen. A shadow of the key is formed on the cardboard screen (see Figure).
In this activity, the key acts as an obstacle in the path of light of the torch. Because of this, light rays of the torch do not go behind the key. So, the part of cardboard screen behind the key does not get any light and remains dark. This dark region on the screen is the shadow of the key.
Since the light coming from the torch travels in straight line path, it cannot bend round the corners of the key and go behind it. Thus, the shadow marks the area where the light of the torch cannot reach because it has been stopped by the key. Please note that the shadow of the key is of the same shape as the key itself. The size of shadow may, however, be different. Here the key is golden in colour but its shadow is black.
If we go on increasing the distance of cardboard screen from the key, we will find that the shadow of the key becomes fainter and fainter. And when the screen is at a large distance from the key, then no shadow of the key is seen on the screen. We can now understand why we do not see the shadows of birds and aeroplanes flying high in the air.
When a bird is on the ground, we can see its shadow which is formed by sunlight. In this case, the bird obstructs the sunlight to form shadow on the nearby ground. When the same bird is flying high up in the air, even then the bird obstructs the sunlight falling on it but its shadow is not seen on the ground because the ground is very, very far below the bird.
Similarly, when an aeroplane is parked on the ground, we can see its shadow which is formed by sunlight. In this case the aeroplane obstructs the sunlight to form a shadow on the nearby ground. When the same aeroplane is flying high up in the air, even then the aeroplane obstructs the sunlight falling on it but its shadow is not seen on the ground because the ground is very, very far below the aeroplane. If, however, a bird or an aeroplane are flying at a very low height, then their shadows can be seen on the ground.
Sometimes the shadows of opaque objects are not seen clearly. For example, though our shadow and that of a ceiling fan formed by an electric bulb or a lighted candle can be seen clearly but the shadows formed by a fluorescent tube-light cannot be seen clearly.
This happens as follows : An electric bulb and a candle form sharp shadows because they are comparatively small sources of light. A fluorescent tube-light does not form sharp shadows because it is a quite large source of light. Since a fluorescent-tube light is very long, therefore, some of the light from it always reaches behind the opaque objects leading to the formation of very faint shadows. In fact, the shadows formed by fluorescent tube-lights are so faint that it is usually very difficult to see them.
It should be noted that a shadow is formed only when there is a source of light and an opaque object obstructs the path of light. So, no shadow can be formed when there is no source of light as on a dark moon less night or in a dark room. Another point to be noted is that whatever be the colour of the object, its shadow is always black.
The shadow of an object has the following characteristics :
- The shadow of an object is erect (same side up as the object).
- The shadow of an object is real (because it can be formed on a screen).
- Irrespective of the colour of the object, the shadow is always black.
- The shadow can be smaller than the object, equal to the object or bigger than the object.
The two important differences between the pinhole images and the shadows are as follows :
(a) The pinhole image of an object is inverted (wrong side up) whereas the shadow of an object is erect (right side up).
(b) The pinhole image of an object is of the same colour as the object but the shadow of an object is always black.
A very important point to remember is that the pinhole images and shadows are possible only if light travels in a straight line path. Thus, the formation of images in a pinhole camera and the formation of shadows by objects placed in the path of light provides the evidence that light travels in straight line path.