Past exam of the physics course of the year 1 of the natural sciences course of the University of Cambridge / 2022 / Section A / 4 / Solution

Take the thin lens at $x=0$, with the object on the left and light rays travelling to the right. The positive object distance is $u=12\mathrm{cm}$ and the focal length is $f=8\mathrm{cm}$. The thin lens equation givesThe transverse magnification and signed image height areThus the real image is $24\mathrm{cm}$ beyond the lens, inverted and $10\mathrm{cm}$ high. For the ray tracing, a light ray parallel to the optical axis emerges through the far optical focus; a light ray through the optical centre continues straight in the ideal thin lens model. They intersect at $(24,-10)\mathrm{cm}$, independently reading off both answers from the diagram.

The calculation and diagram use the ideal paraxial approximation implicit in the thin lens equation. The comparatively large object height does not supply enough information to calculate optical aberrations of a real lens.