Here's an ambulance in Old Town, Swindon. AMBULANCE is written back-to-front on the bonnet so that it can be read in car mirrors. The way that mirrors flip things back-to-front is called lateral inversion. So how come they don't put the writing upside down? That's because rays that are travelling horizontally into the mirror are reflected back horizontally. The top ray and the bottom ray can't cross over as they would for a less or for a concave mirror. In fact, lateral inversion is not caused by the left and right rays crossing over either. It's basically an optical illusion. If you look at your reflection in the mirror, it has to be a reflection of the front of you. The rays can't cross over left and right, so the ray on your left will come from what would be the right hand of your reflection.
