A mirror is a reflective surface that is smooth enough to be able to form an image. The best known example is the plane mirror that most people have at home. In it, 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 erroneously described in high school physics text books.
Early mirrors consisted of a plate or sheet of polished metal, often silver when the reflected image was for viewing (such as for personal grooming) but also of other metals when only the intensity of reflected light was important.
Modern mirrors usually consist of a thin layer of aluminium (or sometimes other metals) deposited on a sheet of glass. They are usually 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. This type of mirror reflects about 80% of the incident light. Front silvered mirrors, where the reflecting surface is placed on the front surface of the glass, have a better image quality but are easily scratched and damaged. They reflect 90% to 95% of the incident light. Astronomical mirrors are of the latter type, and they have to be resurfaced every now and then to keep their quality.
The reflectivity of the mirror coating depends somewhat on the wavelength of light, and this is exploited in some optical work to make cold and hot mirrors. A cold mirror is made by using a transparent substrate and choosing a coating material that is more reflective to visible light and more transmisive to infrared light. A hot mirror is the opposite, the coating preferentially reflects infrared. Mirror surfaces are sometimes given thin film overcoatings both to retard degradation of the surface and to increase their reflectivity in parts of the spectrum where they will be used. For instance, aluminum mirrors are commonly coated with magnesium fluoride. The reflectivity as a function of wavelength depends on both the thickness of the coating and on how it is applied.
For scientific optical work, dielectric mirrors are often used. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. The best mirrors of this type can reflect >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror. Such mirrors are often used in lasers.
A beam of light reflects off of 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.
Mirrors do not actually reverse left and right.
Rear-view mirrors are applied in and on vehicles.
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.
A one-way mirror, also called two-way mirror (!), reflects about half of the light and lets the other half 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. It is used between a dark room and a brightly lit room. From the dark side it looks like a transparent window and from the brightly lit side like a mirror. It may be used to observe criminal suspects, customers (to watch out for theft), etc. The same type of mirror, when used in an optical instrument, is called a half-silvered mirror. 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.
- Note that a one way mirror that only allows light to pass in one direction does not exist. It would allow energy to flow from cold objects to hot objects in contradiction to the laws of thermodynamics. Imagine a cold room separated from a hot room by such a mirror. The mirror reflects the energy from the hot room back inwards, while allowing the energy from the cold room to pass into the hot room. You could run an engine (or thermocouple) off the temperature difference. The result would be free energy, perpetual motion, reversal of entropy!
See also: periscope.