The Mirror
A mirror is an object with a surface that has good specular reflection;
that is, it is smooth enough to form an image
A mirror is an object with a surface that has good specular reflection; that is, it is smooth enough to form
an image. The most familiar type of mirror is the plane mirror, which has a
flat surface. Curved mirrors are also used, to produce magnified or demagnified images or focus light.
Mirrors are most commonly used for personal grooming,
decoration, and architecture. Mirrors are also used in scientific apparatus
such as telescopes and lasers, cameras, and industrial machinery. Most mirrors
are designed for visible light, however, mirrors
designed for other wavelengths of electromagnetic radiation are also used,
especially in optical instruments.
In a plane mirror, a parallel beam of light changes its
direction as a whole, while 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 concave mirrors, where a parallel beam of light
becomes a convergent beam, whose rays intersect in the focus of the mirror.
Lastly, there are convex mirrors, where a parallel beam becomes divergent, with
the rays appearing to diverge from a common intersection "behind" the
mirror. Spherical concave and convex mirrors do not focus parallel rays to a
single point due to spherical aberration. However, the ideal of focusing to a
point is a commonly-used approximation. Parabolic reflectors resolve this,
allowing incoming parallel rays (for example, light from a distant star) to be
focused to a small spot; almost an ideal point. Parabolic reflectors are not
suitable for imaging nearby objects because the light rays are not parallel.
A beam of light reflects off a mirror at an angle of
reflection that is equal to its angle of incidence (if the size of a mirror is
much larger than the wavelength of light). 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.
This law mathematically follows from the interference of a
plane wave on a flat boundary (of much larger size than the wavelength).
Composition
Early mirrors were often little more than a sheet of
polished metal, often silver or copper, for example the Aranmula
kannadi. Most modern mirrors consist of a thin layer
of aluminium deposited on a sheet of glass. This
layer is called the Tain. They are back silvered,
where the reflecting surface is viewed through the glass sheet; this makes the
mirror durable, but lowers the image quality of the mirror due to extraneous
reflections from the front surface of the glass (ordinary glass typically
reflects around 4% of the light). This type of mirror reflects about 80% of the
incident light. The "back side" of the mirror is often painted or
coated in some way to completely seal the metal from corrosion.
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