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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 or simply distort the reflected image.

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.

Effects

In this diagram plain waves reflect off a parabolic mirror to form waves converging onto a focal point.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).

[edit] 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.

Mirrors for precision optical applications are more likely to have the reflective coating on the front surface of the mirror, to eliminate reflection from the glass. Metal films on the front surface are generally covered with a thin, transparent coating to protect them from corrosion. This is often made of silica. In some cases this coating may also enhance reflectivity.

Mirrors designed for special applications, such as in lasers and other advanced optical devices, use a reflective optical coating composed of many layers of different dielectric materials. Such coatings can be designed to have extremely high reflectivity and are reasonably durable. Since they absorb very little of the incident light they can be used with high power lasers without absorbing the energy and being damaged

Applications

Reflections in a spherical convex mirror. The photographer is seen at top right

[edit] Safety and easier viewing

Rear-view mirrors are widely used in and on vehicles (such as automobiles, or bicycles), to allow drivers to see other vehicles coming up behind them. Some motorcycle helmets have a built-in so-called MROS (Multiple Reflective Optic System): a set of reflective surfaces inside the helmet which together function as a rear-view mirror.[1] There exist rear view sunglasses, of which the left end of the left glass and the right end of the right glass work as mirrors.

Convex mirrors are used to provide a wider field of view than a flat mirror. They are sometimes placed at road junctions, and corners of places such as parking lots to allow people to see around corners to avoid crashing into other vehicles or shopping carts. They are also sometimes used as part of security systems, so that a single video camera can show more than one angle at a time.

Corner reflectors use three flat mirrors to reflect light back towards its source. They are used for emergency location, and even laser ranging to the Moon.

With the sun as light source, a mirror can be used to signal, by variations in the orientation of the mirror. The signal can be used over long distances, possibly up to 60 kilometres on a clear day. This technique was used by Native American tribes and numerous militaries to transmit information between distant outposts.

Mirrors can also be used for rescue, especially to attract the attention of search and rescue helicopters. Specialised signalling mirrors are available and are often included in military survival kits.

[edit] Two-way mirrors

A two-way mirror, also sometimes referred to as a one-way mirror or One-way Glass, reflects some percentage of the light and lets some other percentage pass. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, allowing some of the light through the surface (from both sides). It is used between a dark room and a brightly lit room. People on the brightly lit side see their own reflection - it looks like a normal mirror. People on the dark side see through it - it looks like a transparent window. It may be used to observe criminal suspects or customers (to watch out for theft). The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Its purpose is to split a beam of light so that half passes straight through, while the other half is reflected — this is useful for interferometry. The reality television programme Big Brother makes extensive use of two-way mirrors throughout its set (known as the "house") to allow cameramen in special black hallways to use movable cameras to videotape contestants without them coming in contact with the workers. This is due to the rule that contestants are not allowed contact with the outside world.

Contrary to popular belief, passive one-way mirrors that operate directionally between equally lit rooms do not exist. The laws of physics do not allow for real, passive one-way mirrors or windows (ones that do not need external energy); if such a device were possible, one could break the second law of thermodynamics and make energy flow from a cold object to a hot one, by placing such a mirror between them. One-way windows can be made to work with polarized light, however, without violating the second law.[1][2] Optical isolators are one-way devices that are commonly used with lasers.

I would venture that a 1.3-1.5 m mirror would do (maybe a little smaller) if she was back a metre or so. I have a 1.6m mirror and can see myself from just .5 m and i'm near 1.9 m.

Height off the floor also plays a large part in what you can see in the mirror. Therefore placement should be a little lower than the top of the head and then you get full body view.

80 cm mirror will do. I am sorry I cn not draw the diagram in the Yahoo Answers

1.6 m...law of reflection..

I don't understand them at all as mine didn't break when i looked in it. l.o.l.................................