Past exam of the physics 2 paper of the University of Oxford / 2016 / 10 / b / Solution

For a real image, $u>f$ and $v=fu/(u-f)>0$. The object-to-image separation isIts excess above $4f$ factors into a nonnegative expression:ThereforeAt this minimum the image is inverted and has unit absolute magnification. The restriction to a real object and a real image matters: the positive separation used here is not the same quantity as a signed distance to a virtual image.