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. For example, during a lunar eclipse, we see a part of the earth’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. 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. 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 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.

Making a Pinhole Camera

  • Take two rectangular boxes that fit into one another without leaving any gap.
  • Cut open one side of each box.
  • Make a small hole in the larger box at the centre of the closed end opposite the side that has been cut open.
  • Cut a square of side five centimetres in the smaller box in the closed end opposite the side that has been cut open. Cover this square with tracing paper.
  • 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.
  • 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.

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