Mirrors

A mirror is a surface with good specular reflection that is smooth enough to form an image. The best known example is the plane mirror. more...

The most common use is in the home for personal grooming but mirrors are also used in scientific apparatus such as telescopes and lasers, and in industrial machinery.

Effect

In a plane mirror, a parallel beam of light changes its direction as a whole, whilst still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object (see mirror image). There are also parabolic concave mirrors, where a parallel beam of light becomes a convergent beam, whose rays intersect in the focus of the mirror. Finally, there are convex mirrors, where a parallel beam becomes divergent, with the apparent intersection occurring behind the mirror. Note that spherical concave and convex mirrors do not have a single focal point, as often described in high school physics text books (see spherical aberration in lens (optics) and aberration in optical systems).

A beam of light reflects off a mirror at an angle of reflection that is equal to its angle of incidence. That is, if the beam of light is shining on a mirror's surface at a 30° angle from vertical, then it reflects from the point of incidence at a 30° angle from vertical in the opposite direction.

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.

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