Lenses and images

The bench, lenses, lamp box, window and screen are simulated. Both lenses are thin, 5.0 cm across, with a focal length of 10 cm, and the images follow the same rules as ray diagrams. The screen moves in 1 cm steps, so “sharp” means the step nearest the image. Out of focus, each point of the image is drawn spread over a patch as wide as the cone of light reaching the screen.

A real lens is not perfectly thin, so its image is never perfectly sharp at the edges, and its focal length is a little different for different colours. The rays are drawn in one plane through the principal axis, but light passes through the whole lens. The window is treated as very far away: about 20 m away, its sharp picture would be only half a millimetre further than 10 cm from the lens, which a 1 cm step cannot show.

With reduced motion each step happens at once: the lamp, the lenses, the screen and the rays appear straight away as they are at the end of the step.