Past exam of the physics course of the year 1 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / A5 / Solution

For the real object and real image, take positive distances $u=f+u^{\prime }$ and $v=f+v^{\prime }$. The thin lens equation yieldsThis is Newton's lens formula. The distances $u^{\prime },v^{\prime }$ refer to the adjacent focal points, not the lens plane.