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

The image is upright vertically when viewed down from behind, but left and right are interchanged. To track the image formation, let $+x$ point forwards and $+z$ upwards. Incoming light travels along $-x$. The $45^{\circ }$ flip plane mirror sends it along $+z$. Before this reflection, the optical lens has inverted the scene's top into the $-z$ direction; the law of reflection maps that direction into $+x$ on the horizontal screen. The screen's far edge is the top of the picture for someone looking down from behind. The horizontal inversion from the optical lens remains, since this plane mirror does not change the lateral coordinate.