In geometry, the mirror image of an object or two-dimensional figure is the virtual image formed by reflection in a plane mirror; it is of the same size as the original object, yet different, unless the object or figure has reflection symmetry (also known in the terminology of modern physics as a P-symmetry).

If a point of an object has coordinates (x,y,z) then the image of this point (as reflected from the mirror in y,z plane) has coordinates (-x,y,z) – so mirror reflection is a reversal of the coordinate axis perpendicular to the mirror’s surface. Thus, a mirror image does not have reversed right and left (or up and down), but rather reversed front and back.

In Three Dimensions

The concept of mirror image can be extended to three-dimensional objects, including the inside parts, even if they are not transparent. The term then relates to structural as well as visual aspects. This is also called enantiomer or enantiomorph.

A mirror image appears three-dimensional if the observer moves. This is because the relative position of objects changes as the observer’s perspective changes. [1]

Looking through a mirror from different positions (but necessarily with the point of observation restricted to the halfspace on one side of the mirror) is like looking at the 3D mirror image of space; without further mirrors only the mirror image of the halfspace before the mirror is relevant; if there is another mirror, the mirror image of the other halfspace is too.