Mirror

A mirror is a sufficiently polished surface so that a image is formed there by reflection. It is often a fine metal layer, which, to be protected, is placed under a glass plate for the domestic mirrors (the mirrors used in the optical instruments comprise the metal face with the top, glass being only one support of stable mechanical quality).

The relative adjective to the mirror is specular because it makes it possible to see (like the Speculum the fact also).

One can see oneself by using the Reflet at water surface (like Narcisse) or in a pane; in this case the reflection is partial while with a perfect mirror the reflection is total.

One can also obtain total reflection when a ray passes from a medium of high Index of refraction about a weak middle of index, under a shaving incidence; for example when a ray passes from water in the air, or of glass in the air. This property is used for the prism S with total reflection.

When it is said that the surface of a mirror must be polished, that means that no defect is acceptable so that all the reflection of the incidental wave is done in the desired direction. The size of the visible defect is about the Wavelength of the electromagnetic Onde. Thus, with the visible light, the defects must be smaller than 0,01  μm, which is very forcing technically. On the other hand, with the waves used by television, the defect must be smaller than 0,1  mm only, which explains why the parabolas of television (which are mirrors making it possible to concentrate, to focus the waves emitted by the satellites) are rough with the eye and under the hand; they are on the other hand perfectly smooth for the Hertzian waves. For the radars, the order of magnitude of the allowable defect is about cm, one can thus use a netting like mirror; It is just as for some Radiotélescope S.

Use of the mirrors

The first use of the mirrors was undoubtedly to look at oneself (coquettery).

Archimedes, between 215 and 212 av. J. - C., would have made use of concave mirrors to concentrate the rays of the Sun and to ignite the veils of the Roman ships which attacked Syracuse (“burning mirror”). This property of focusing is used nowadays in the Télescope S like for the Solar furnace of Odeillo.

The human field of view is limited. By reflecting the rays coming from another direction, a mirror makes it possible to extend this field of view in other directions (but it masks part of the direct field of view); it is the principle of the Rétroviseur S of Automobile.

A mirror placed well makes it possible to see behind an object; for example, the hairdresser puts a mirror behind the head of the customer so that this one can see, in the mirror facing him, the cut seen of behind. The dentists use a small mirror at the end of a stem to see the back of the teeth. The security services, organizes or customs can inspect the lower part of a vehicle, a low piece of furniture or the top of a cupboard with a similar system.

The purpose of the mirrors evoked above are to give a representation faithful (or slightly deformed but increased) of the objects. But a mirror can also give a voluntarily deformed vision, for example in the deforming mirrors of attractions of fair.

The Anamorphose S with mirror make it possible thanks to the interposition of a cylindrical or conical mirror to reveal an image which is the reflection of a deformed image conceived for this purpose. The deformed image is painted on a plane surface around a site envisaged of the mirror; it is only by installing the mirror there that the image appears not deformed on the surface of the mirror. Spread 17th and the 18th century, this process made it possible to diffuse Caricature S, erotic scenes and scatologic, scenes of Sorcellerie and grotesque which appeared for a confidential public when the mirror was well positioned on painting.

The mirrors reflect the rays in a symmetrical way, law described by Descartes at the 17th century and reverse the image (right/left). Thus, if one sees an object, from orientation of mirror (angle which its surface with the axis of vision makes), one can determine the direction in whom the object observed is. This principle is used in the Sextant S to determine the height of a star (angle compared to the horizon), and by the Géomètre S to determine the distances.

A mirror can reflect a luminous ray towards the place from where it comes, after having traversed a certain distance, but the engineering problems to be solved will be that of alignment. The time of way of the light was thus used for measurement of the Speed of light. This speed being known, one can make use of this technique to determine the distances; for example, one measured the distance Ground - the Moon - Ground using a beam Laser emitted since the Earth, reflected by a trihedral block of mirrors placed on the Moon by a mission Apollo and returning then towards his source.

A semi-transparent mirror can separate a ray from light in two identical rays. These two rays having thereafter a different way, the final difference between the rays makes it possible to know the difference between the crossed mediums or the objects met. Thus by taking one of the rays like reference, one can build a Hologramme (in the case of a beam of coherent Lumière). This technique is also used in Interférométrie; if that can be used in practical works as optics to learn how to know the properties of the light, the interferometers made it possible Albert Einstein to deduce invariance speed of light (Expérience of Michelson-Morley), and one makes use of it to study the gravitational waves.

If a ray is reflected and makes a return ticket, he traverses a double distance; using mirrors, one can thus have a great optical way in a reduced space. This is used for certain experiments of optics, like having compact Télescope S. If a ray is trapped between two mirrors, that creates a cavity resounding which makes it possible to select the wavelengths; this is used in the Laser and makes it possible to have a monochromatic and coherent light.

It will be noted that one makes use of the Test of the mirror in Zoologie and Philosophie of the spirit to appreciate the degree of Conscience of oneself of which make different proof the S Animal are.

Symbolic system of the mirrors

The plane mirror returns an image faithful (but reversed) of the person who looks at herself inside; it is thus charged with strong a connotation symbolic system. It makes it possible to be seen such as one is, but always under a single angle (face to face and reversed), in particular with its defects. It is often associated with the truth, such as for example the Magic Miroir of White-Snow .

The mirror is also the reverser of the truth. In Don Quichotte , the Knight of the Mirrors is the mortal enemy of Hidalgo of which he disavows the inspiration.

It is also a strong symbol of mythology Japan ease, and Shintoïsme; it is also one of the attributes of the Japanese goddess of the Sun Amaterasu.

The psychoanalyst Jacques Lacan defines the “mirror stage”, which is the moment when the child is aware that he sees itself in a mirror. This self-knowledge (and not this recognition, since before seeing itself in a mirror, a newborn does not know the appearance of his face) intervene between 18 and 24 months, in general. She takes part of the installation of various feelings, such as the Empathie, the Fierté, the Honte and the Inversion (appearance of the negation).

It is also the symbol of a carries, of a limit towards another world, particularly emphasized in Alice at the country of the wonders.

Realization practices mirrors

The first mirrors were manufactured by polishing Métal. To avoid the problems of Oxidation metal while keeping a good rigidity, one now in general uses a fine metal sheeting joined under a plate of Verre.

In this last case, there is a phenomenon of reflection (known as partial) and refraction in the glass plate before reaching the metal sheet, protected with the back by a coating called Tain, from where birth of two very unequal images of intensity, of which weakest is perceptible all the same but which, in the everyday usage, does not have importance, our brain eliminating it parasitic under image.
En photographs, the mirrors, used to make stereotypes with 90° (to preserve more naturalness at the subject photographed) are of " truths miroirs" thus without interposed ice. In the same way in the experiments of optics (interferences with the mirrors of Fresnel, for example), one uses mirrors known as " face avant" , in which glass is used only as stable mechanical stand.

Internal bonds

External bonds

  • Some mirrors in Western Painting

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