Mirror: Encyclopedia II - Mirror - Image in a mirror

Mirror - Image in a mirror

For an object with approximate reflection symmetry, a reflection in some mirror plane corresponds to a combination of:

• a translation if the mirror is parallel to the symmetry plane of the object, and otherwise a rotation about the line of intersection of the two planes by an angle which is twice the angle between the two planes
• a reflection in the approximate symmetry plane of the object (due to the assumption this is a minor change)

We can apply this to the image in a mirror of, say, a standing person, because people have approximate bilateral symmetry. The image is the most realistic if it is still vertical, i.e., if the rotation is about a vertical axis. This is the case iff the mirror is vertical. In this case the image of the person is in normal standing orientation and vertically in a normal position, at a horizontally different position and with an orientation rotated about a vertical axis, the latter except if the mirror is parallel to the approximate symmetry plane of the person.

In particular, if one looks at one's image in a vertical mirror in left-right orientation, the image corresponds to a rotation by 180° about the vertical axis in the mirror, combined with a reflection in one's approximate symmetry plane.

When the rotation is so obvious that it is not worth mentioning, the second component in this decomposition of the effect of the mirror is sometimes emphasized, by saying that a mirror "reverses left and right".

Adapted from the Wikipedia article "Image in a mirror", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki