Mirrors are made by coating the back of a sheet of glass with an alloy of mercury and another metal. This means that light does not pass through the glass, but is bounced back to give a reflection.
The nature of modern mirrors is not fundamentally different from a pool of water. When light strikes any surface, some of it will be reflected. Mirrors are simply smooth surfaces with shiny, dark backgrounds that reflect very well. Water reflects well, glass reflects poorly, and polished metal reflects extremely well. The degree of reflectivity—how much light bounces off of a surface—and the diffusivity of a surface—what direction light bounces off of a surface—may be altered. These alterations are merely refinements, however. In general, all reflective surfaces, and hence, all mirrors, are really the same in character.
Man-made mirrors have been in existence since ancient times. The first mirrors were often sheets of polished metal and were used almost exclusively by the ruling classes. Appearance often reflected, and in some cases determined, position and power in society, so the demand for looking glasses was high, as was the demand for the improvement of mirror-making techniques. Silvering—the process of coating the back of a glass sheet with melted silver—became the most popular method for making mirrors in the 1600s. The glass used in these early mirrors was often warped, creating a ripple in the image. In some severe cases, the images these mirrors reflected were similar to those we’d see in a fun-house mirror today. Modern glassmaking and metallurgical techniques make it easy to produce sheets of glass that are very flat and uniformly coated on the back, improving image clarity tremendously. Still, the quality of a mirror depends on the time and materials expended to make it. A handheld purse mirror may reflect a distorted image, while a good bathroom mirror will probably have no noticeable distortions. Scientific mirrors are designed with virtually no imperfections or distorting qualities whatsoever.
Materials technology drastically affects the quality of a mirror. Light reflects best from surfaces that are non-diffusive, that is, smooth and opaque, rather than transparent. Any flaw in this arrangement will detract from the effectiveness of the mirror. Innovations in mirror making have been directed towards flattening the glass used and applying metal coatings of uniform thickness, because light traveling through different thicknesses of glass over different parts of a mirror results in a distorted image. It is due to these irregularities that some mirrors make you look thinner and some fatter than normal. If the metal backing on a mirror is scratched or thin in spots, the brightness of the reflection will also be uneven. If the coating is very thin, it may be possible to see through the mirror. This is how one-way mirrors are made. Non-opaque coating is layered over the thin, metal backing and only one side of the mirror (the reflecting side) is lit. This allows a viewer on the other side, in a darkened room, to see through.
Glass, the main component of mirrors, is a poor reflector. It reflects only about 4 percent of the light which strikes it. It does, however, possess the property of uniformity, particularly when polished. This means that the glass contains very few pits after polishing and will form an effective base for a reflective layer of metal. When the metal layer is deposited, the surface is very even, with no bumps or wells. Glass is also considered a good material for mirrors because it can be molded into various shapes for specialty mirrors. Glass sheets are made from silica, which can be mined or refined from sand. Glass made from natural crystals of silica is known as fused quartz. There are also synthetic glasses, which are referred to as synthetic fused silica. The silica, or quartz, is melted to high temperatures, and poured or rolled out into sheets.
A few other types of glass are used for high-quality scientific grade mirrors. These usually contain some other chemical component to strengthen the glass or make it resistant to certain environmental extremes. Pyrex, for example, is a borosilicate glass—a glass composed of silica and boron—that is used when mirrors must withstand high temperatures.
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