Mirror image

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

• The mirror-image of b is d or p (d and p are equal except for orientation).
• The tetrominos S and Z

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. Chirality is a term in geometry where a figure is said to be chiral if it is not identical to its mirror image, or, more precisely, can't be mapped to its mirror image by rotations and translations alone. Examples:

• The mirror-image of a right hand is like a left hand.
• The mirror-image of a human body is roughly like a human body, but with internal differences; for a particular person there are also minor external differences, the most striking may be the hair if that is not symmetrically cut and combed.
• The 3D tetrominos left and right screw
• The two versions of the snub cube are mirror images of each other.
• The two isomers in the case of optical isomerism.
• Clothing is often approximately symmetric. However, for fastening the right to the left side (of a shirt, coat, flies, dress, etc.), often on the front side, but also on the back side, if buttons are used one side goes over the other; in the case of a zip fastener there is at least minor asymmetry, but often also one flap goes over the other. A belt, although often vertically symmetric, provides also asymmetry. A skirt may have a zip fastener on one side. There may also be asymmetry with regard to pockets, for example only one inner pocket.

A mirror image of a two-dimensional figure is also obtained when looking at it from the other side, in the case that the figure can still be seen from there. This may be the case due to transparency, or if the coloring is not just at the surface but also inside, or if a shape has been cut out. Examples:

• text or pictures on glass or textiles (a printed T-shirt worn inside out, a parasol)
• paper printed on one side, looked at from the other side, holding it to the light

The mirror-image of a mirror image is a regular image. When you see a reflection that surprisingly is a regular image this is usually caused by the fact that you are looking at the reflection of a reflection, or the reflection of an image seen from the other side (see above). On sunny days perhaps the most common example of the latter is seeing the reflection in a window of the inside of a parasol with text on it. Occasionally you can see a mirror image, even though you are aware of looking at the reflection of a reflection; this may be due to a third reflection. A text is sometimes deliberately displayed in mirror image, in order to be read through a mirror, e.g:

• on the front side of a car, to be seen as a regular text in the rear view mirror of the car in front
• at the back of a movie theater, captions to be read by hearing impaired, through a special reflecting panel (Rear Window Captioning System)

A special kind of "mirror image" of a text is with the letters in reversed order, while the individual letters are normal, see http://wikipedia.org.mirror.sytes.org/wiki/mirror_image . The real mirror image of that is with is with the letters in normal order, but each in mirror image. See also Chirality, Left and right, handedness

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