Objectives
After working through this chapter, you should be able to:
Explain how a pinhole produces an image
Create your own camera obscura
Introduction
As we discussed very briefly in chapter 13 , a ‘camera obscura’ is an imaging device that utilises the theory behind pinhole photography. When light emanating from an object shines through a pinhole, it produces an upside-down image. This is because light rays from the tip of the object (e.g. the top of the mug in Fig. 18.1 ) travel in a straight line through the pinhole and end up near the floor, whilst light rays from the base of the object travel in a straight line through the pinhole and end up near the ceiling. When these rays form a focus, it necessarily produces an inverted image.
The experiment
In order to see how this works, it’s best to make our own. To do this, we’ll need a pinhole aperture and something to project the image onto ( Fig. 18.2 ).
Equipment required
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A pencil
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A drawing pin/needle/something with which to make a very small hole
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Two pieces of A4-sized black card (if you don’t have black card, a cereal box might do)
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Tape of some kind
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Scissors
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Tracing paper or equivalently translucent material that, when held up to a lamp, is diffusely illuminated but does not show a clear image of the lamp through the material (e.g. a translucent sandwich bag or the bag from inside a cereal box – if these are too transparent, you can double up the layers)
Method
- 1.
Cut an 8 cm strip off the first piece of black card as shown in Fig. 18.3 . Set the 8 cm strip to one side for now.
• Fig. 18.3
Diagram of step 1 of method – cut an 8 cm strip off one of the pieces of card.
- 2.
Take the remaining part of the first piece of black card and roll it up to make a tube (tube 1 in Fig. 18.4 ). Tape to secure. The diameter of the tube should be roughly 6 cm, so overlapping is allowed.
• Fig. 18.4
There should be two tubes – a short, wide one (tube 1) and a narrow, long one (tube 2).
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