Two mirrors at a 90 degree angle form a corner reflector and it has the property that light striking it will be reflected back in the same direction parallel to the source light. This is the same principle behind bicycle reflectors. Because there is a gap between the two mirrors, there will be a minimum distance between an incoming lightbeam and the parallel outgoing lightbeam. If this distance (shown as d) is larger than the distance of one eye to the opposite side of my face, then I will not be able to see my face, now matter how I turn my head (unless I stare directly into one of the mirrors).
