Past exam of the physics 2 paper of the University of Oxford / 2021 resit / 2 / c

The minimum is 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.