This strange thing on the picture is the rear reflector of the back of a
car. Contrary to a normal mirror, it reflects the light always in exactly the
same direction, it comes from. How this works can be seen on the sketch on
the right. Two mirror planes meeting with a right angle are necessary. In
practice (i.e. in three dimensions) even at least three mirror planes are
necessary, because with two planes, the component of the light parallel to
these two would not be reversed. For example the three coordinate planes
could be used.