= Solution
The minimum is \b[two additional <plane mirrors>], making three including the flip <plane mirror>. Their reflecting planes can meet at $45^\circ$ in the vertical section, as in a <pentaprism>. For example, in the $xz$ section take the first added reflecting plane at $67.5^\circ$ to $+x$ and the second at $112.5^\circ$. The upward beam first travels diagonally forwards and upwards, then horizontally backwards towards an eye looking forwards. The two <reflections> together rotate directions through $90^\circ$: the propagation direction $+z$ becomes $-x$, while the screen's top direction $+x$ becomes $+z$ at the eye. Thus the picture remains upright vertically.
One additional <plane mirror> would also send $+z$ into $-x$, but it would send the top direction $+x$ into $-z$, making the image upside down. This proves minimality for ordinary <plane mirrors>. The three-<reflection> arrangement still reverses left and right; correcting that additionally requires a roof arrangement or another suitable image-erecting system. Vertical erection alone is what is asked here.
\Image[/past-exam-of-the-physics-2-paper-of-the-university-of-oxford/2021-resit-viewfinder-mirrors.png]
{title=An original vertical-section ray construction with the minimum two added plane mirrors}
Back to article page
