Model of a camera
A simple model of a camera, demonstrating image formation, focus and the effect of aperture on depth of field.
Apparatus and materials
For each student or group of students
Lamp, stand, housing (one left- and one right-handed), 2
Holder for slits
Plano-cylindrical lens, approximately + 7 D
Power supply for lamp
A4 paper, plain white, one sheet
Health & Safety and Technical notes
Warn students that their lamps may become hot if they are left on for a series of experiments of this type.
Show students how to stand the cylindrical lens on its base, with the multiple slit about 10 cm in front of it. You might then give them these instructions:
Put the sheet of paper at one end of your table. Your model camera will be there. Place your lamp about 1 metre from the paper. Raise or lower it until light from it shines right across the paper. Place your lens, upright, about 10 centimetres from the end of the paper nearest the lamp. Mount the multiple slit at the end of the paper so that rays of light come shining through it to the lens. Can you see the image of the lamp where all the rays meet? Draw an outline of a camera box on the paper with a front opening where the lens is, and a place for the 'film' at the back where the image is, as in the lower diagram above.
Place a second lamp beside the first, with its beam also shining through the multiple slit. You can see how two object-points make two image-points where the film should be. You might imagine these are the head and toes of a person being photographed.
Now move one of the two lamps nearer the camera. See how you get a patch on the 'film' instead of a point. A patch like that for each point of the original object would make an 'out-of-focus' picture. Keep the two lamps where they are, at unequal distances. Make the front opening of your lens narrower by pushing in some small barriers to stop the outer rays from getting through. See what that does to the 'out of focus' patch on the 'film'.
1 The lamp needs to be carefully positioned to enable the beam to show up across 1 m of the bench top.
2 When the second lamp is moved towards the lens, the focus of its beam of rays will be further from the lens, and thus form a 'patch' where students have drawn the 'film'.
3 Forming a smaller aperture with the barriers will increase the camera's depth of field, so that the 'out of focus' effect will be reduced.
This experiment was safety-checked in January 2007