After working through this chapter, you should be able to:
Explain how a pinhole produces an image
Create your own camera obscura
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.
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 ).
A drawing pin/needle/something with which to make a very small hole
Two pieces of A4-sized black card (if you don’t have black card, a cereal box might do)
Tape of some kind
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)
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.
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.