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Shadow A shadow is a dark patch formed behind an opaque object when it is placed in the path of light. A shadow is formed only when a light source, an opaque object and a screen are present. For example, during a lunar eclipse, we see a part of the earthâ€™s shadow on the surface of the moon. This happens when the earth, the sun and the moon are in a straight line, with the earth between the sun and the moon. Here, the sun acts as the light source, the earth as the opaque object, and the moon as the screen. Shadows are formed due to the rectilinear propagation of light. The size and shape of the shadow depends on the position and orientation of the opaque object between the source of light and the screen.  Whatever be the colour of the object, its shadow is always black because the shadow region  is not illuminated by light. The length and shape of a shadow depends on the object by which it is formed. In the olden days, shadows caused by objects placed in the sun were used to measure time. Such a device is called a sun dial. The Jantar Mantar in Jaipur consists of a sundial or Samrat Yantra, which can be used to tell the time, as its shadow moves visibly at one millimetre per second, or roughly six centimetres every minute. Pinhole Camera The pinhole camera works on the principle that light travels in a straight line. The image formed by a pinhole camera is real, inverted and diminished in size when compared to the original object.  Pinhole cameras are inexpensive and are easy to make. An eclipse can be viewed using a pinhole camera. Making a Pinhole Camera      â€¢  Take two rectangular boxes that fit into one another without leaving any gap.      â€¢  Cut open one side of each box.      â€¢  Make a small hole in the larger box at the centre of the closed end opposite to the side that has been cut open.      â€¢  Cut a square of side five centimetres in the smaller box in the closed end opposite to the side that has been cut open. Cover this square with tracing paper.      â€¢  Finally, slide the smaller box into the larger box, ensuring that the pinhole and the tracing paper are in line with one another, but at the opposite ends.      â€¢  Slide the smaller box to adjust the focus so that you can capture the image of any object you want. This is the basic structure of the pinhole camera. However, the developed form of a pinhole camera uses photosensitive film instead of translucent paper to capture images. The film can be developed later to obtain photo prints. Nowadays, of course, we use digital cameras that store images in a computer chip. However, the basic principle of capturing the image remains the same. Differences between A Shadow and A Pinhole Camera Image      â€¢  A shadow is in general black and does not give the details of the object.  The image formed by a pinhole camera is colourful and contains all the details of the object.      â€¢  A shadow can be bigger than the object whereas the image formed by a pinhole camera is always smaller than the object.      â€¢  The shape of a shadow varies depending on the orientation of the object between the light and screen while the pinhole camera image is always inverted and has the same shape as the object.

#### Summary

Shadow A shadow is a dark patch formed behind an opaque object when it is placed in the path of light. A shadow is formed only when a light source, an opaque object and a screen are present. For example, during a lunar eclipse, we see a part of the earthâ€™s shadow on the surface of the moon. This happens when the earth, the sun and the moon are in a straight line, with the earth between the sun and the moon. Here, the sun acts as the light source, the earth as the opaque object, and the moon as the screen. Shadows are formed due to the rectilinear propagation of light. The size and shape of the shadow depends on the position and orientation of the opaque object between the source of light and the screen.  Whatever be the colour of the object, its shadow is always black because the shadow region  is not illuminated by light. The length and shape of a shadow depends on the object by which it is formed. In the olden days, shadows caused by objects placed in the sun were used to measure time. Such a device is called a sun dial. The Jantar Mantar in Jaipur consists of a sundial or Samrat Yantra, which can be used to tell the time, as its shadow moves visibly at one millimetre per second, or roughly six centimetres every minute. Pinhole Camera The pinhole camera works on the principle that light travels in a straight line. The image formed by a pinhole camera is real, inverted and diminished in size when compared to the original object.  Pinhole cameras are inexpensive and are easy to make. An eclipse can be viewed using a pinhole camera. Making a Pinhole Camera      â€¢  Take two rectangular boxes that fit into one another without leaving any gap.      â€¢  Cut open one side of each box.      â€¢  Make a small hole in the larger box at the centre of the closed end opposite to the side that has been cut open.      â€¢  Cut a square of side five centimetres in the smaller box in the closed end opposite to the side that has been cut open. Cover this square with tracing paper.      â€¢  Finally, slide the smaller box into the larger box, ensuring that the pinhole and the tracing paper are in line with one another, but at the opposite ends.      â€¢  Slide the smaller box to adjust the focus so that you can capture the image of any object you want. This is the basic structure of the pinhole camera. However, the developed form of a pinhole camera uses photosensitive film instead of translucent paper to capture images. The film can be developed later to obtain photo prints. Nowadays, of course, we use digital cameras that store images in a computer chip. However, the basic principle of capturing the image remains the same. Differences between A Shadow and A Pinhole Camera Image      â€¢  A shadow is in general black and does not give the details of the object.  The image formed by a pinhole camera is colourful and contains all the details of the object.      â€¢  A shadow can be bigger than the object whereas the image formed by a pinhole camera is always smaller than the object.      â€¢  The shape of a shadow varies depending on the orientation of the object between the light and screen while the pinhole camera image is always inverted and has the same shape as the object.

#### Activities

 Activity 1 Childrensuniversity has created a virtual activity to demosnstrate the change in the size of the shadow during the day light relative to the position of sun. Got to Actitivity Activity 2 Sciencekids.co.nz has created a virtual interactive simulation to understand the concept of formation of shadows and the charracteristics of the shadows formed by the different objects with repect to the change in relative positions of the object, source of light and the screen. Go to Activity Activity 3 Astro.unl.edu has created a good animation that simulates the phases of moon during its motion around the earth and also describes the concept of formation of shadows  Go to Activity Activity 4 Learnthings.co.za has created a good animation that simulates that transparent and translucent materials let light through, whereas opaque materials do not.  If an opaque material is put in front of a light source, it blocks the light and a shadow forms. A shadow made by the sun will change in length and position throughout the day.   We can explore that the size of the shadow at different times of the day changes by changing the height of the stick by dragging it up and down.  Go to Actitivity

#### References

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