Physics - Geometrical Optics Question with Solution | TestHub

Image length $=\frac{f^2}{u-f}$. If the far end is at $u^{\prime }$, $\frac{f^2}{(u-f)}=\left|\frac{fu}{u-f}-\frac{f{u}^{\prime }}{u^{\prime }-f}\right|$, which is satisfied as $u^{\prime }\to$ infinity (the far end's image approaches the focus while the near end's image stays at finite distance).