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A plane mirror is a
mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
with a flat ( planar) reflective surface. For
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
rays striking a plane mirror, the angle of reflection equals the angle of incidence. The angle of the incidence is the angle between the incident ray and the
surface normal In geometry, a normal is an object such as a line, ray, or vector that is perpendicular to a given object. For example, the normal line to a plane curve at a given point is the (infinite) line perpendicular to the tangent line to the curve ...
(an imaginary line perpendicular to the surface). Therefore, the angle of reflection is the angle between the reflected ray and the normal and a collimated beam of light does not spread out after reflection from a plane mirror, except for
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
effects. A plane mirror makes an image of objects in front of the mirror; these images appear to be behind the plane in which the mirror lies. A straight line drawn from part of an object to the corresponding part of its image makes a right angle with, and is bisected by, the surface of the plane mirror. The image formed by a plane mirror is virtual (meaning that the light rays do not actually come from the image) it is not
real image {{citations needed, date=June 2019 In optics, an ''image'' is defined as the collection of focus points of light rays coming from an object. A real image is the collection of focus points actually made by converging/diverging rays, while a ...
(meaning that the light rays do actually come from the image). it is always upright, and of the same shape and size as the object it is reflecting. A virtual image is a copy of an object formed at the location from which the light rays appear to come. Actually, the image formed in the mirror is a perverted image (
Perversion Perversion is a form of human behavior which deviates from what is considered to be orthodox or normal. Although the term ''perversion'' can refer to a variety of forms of deviation, it is most often used to describe sexual behaviors that are co ...
), there is a misconception among people about having confused with perverted and laterally-inverted image. If a person is reflected in a plane mirror, the image of his right hand appears to be the left hand of the image. Plane mirrors are the only type of mirror for which a object produces an image that is virtual, erect and of the same size as the object in all cases irrespective of the shape, size and distance from mirror of the object however same is possible for other types of mirror (concave and convex) but only for a specific conditions . However the
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative fo ...
of a plane mirror is
infinity Infinity is that which is boundless, endless, or larger than any natural number. It is often denoted by the infinity symbol . Since the time of the ancient Greeks, the philosophical nature of infinity was the subject of many discussions a ...
; its optical power is zero. Using the mirror equation, where d_0 is the object distance, d_i is the image distance, and f is the focal length: \frac+\frac=\frac ''Since'' frac=0/math>, \frac=-\frac -d_0=d_i Concave and Convex mirrors (spherical mirrors) are also able to produce images similar to a plane mirror. However, the images formed by them are not of the same size as the object like they are in a plane mirror in all conditions rather specific one . In a convex mirror, the virtual image formed is always diminished, whereas in a concave mirror when the object is placed between the focus and the pole, an enlarged virtual image is formed. Therefore, in applications where a virtual image of the same size is required, a plane mirror is preferred over spherical mirrors.


Preparation

A plane mirror is made using some highly reflecting and polished surface such as a
silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
or
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ...
surface in a process called
silvering Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal. Process Mo ...
. After silvering, a thin layer of red lead oxide is applied at the back of the mirror. The reflecting surface reflects most of the light striking it as long as the surface remains uncontaminated by
tarnish Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen ...
ing or
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
. Most modern plane mirrors are designed with a thin piece of
plate glass Plate glass, flat glass or sheet glass is a type of glass, initially produced in plane form, commonly used for windows, glass doors, transparent walls, and windscreens. For modern architectural and automotive applications, the flat glass is ...
that protects and strengthens the mirror surface and helps prevent tarnishing. Historically, mirrors were simply flat pieces of
polish Polish may refer to: * Anything from or related to Poland, a country in Europe * Polish language * Poles, people from Poland or of Polish descent * Polish chicken *Polish brothers (Mark Polish and Michael Polish, born 1970), American twin scree ...
ed
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish- ...
, obsidian,
brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wit ...
, or a precious metal. Mirrors made from
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...
also exist, as the elements
gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (alumini ...
and mercury are both highly reflective in their liquid state.


Relation to curved mirrors

Mathematically, a plane mirror can be considered to be the limit of either a concave or a convex spherical
curved mirror A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...
as the radius, and therefore the
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative fo ...
becomes infinity.


See also

*
Geometrical optics Geometrical optics, or ray optics, is a model of optics that describes light propagation in terms of '' rays''. The ray in geometrical optics is an abstraction useful for approximating the paths along which light propagates under certain circumsta ...
*
Specular reflection Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...
* Chinese magic mirror *
Law of reflection Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...


References

{{Reflist Mirrors