When most people use the term "mirror," they are referring to what is known as a plane mirror. A plane mirror takes the light that hits it and reflects it back. Mirrors used for common consumer purposes are of this sort.

A mirror is essentially a highly reflective surface. The sorts of mirrors one sees on walls or in bathrooms are of a type known as back-silvered mirrors. This means that the reflective surface--in most modern mirrors this is aluminum--is viewed through a thin layer of glass. The glass protects the aluminum from scratching and bubbling, but also distorts the image somewhat.

Early mirrors were created by simply polishing a suitable substance until it became highly reflective. Neolithic mirrors have been discovered, made by grinding down obsidian rocks and polishing them to an incredible sheen. These mirrors have remarkable properties, allowing even subtle details to be clearly seen in their reflections.

To make a Neolithic mirror, one need first find the right stone--ideally, obsidian. A rougher stone is used to grind the base stone down to a flat surface on one side. After a flat surface is achieved, a finer grinding stone and clay slip can be used to polish the stone to a fully reflective sheen. Extremely fine abrasives, such as ash, can help further. After a substantial amount of time and effort, one will have created a very primitive form of mirror.

Modern mirrors, however, are made using an entirely different process. By taking liquid metals and allowing them to condense on a sheet of glass, one can get a surface far more reflective than anything achieved by polish. Making a mirror can be done at home with only a few supplies easily acquired at a local chemistry shop.

With pure silver nitrate, distilled water, and ammonia, one can make a mirror virtually indistinguishable from those purchased at a store. The process (in abbreviated form) involves dissolving a small amount of silver nitrate in distilled water, then adding diluted ammonia until the mixture goes through distinct chemical changes. A second mixture is made using silver nitrate and Rochelle salts. This mixture must be boiled and filtered. By pouring these mixtures on to a very clean piece of glass sufficiently heated to the proper temperature, the silver will precipitate and form an even coating on the glass. After drying, one can coat the back of the silver with a solid paint to help prevent degradation of the silver. The result is a fully functional mirror.

Commercial mirrors are manufactured in more or less the same manner as is described above, though materials such as aluminum might be used instead of silver. Mirrors produced for specialized purposes, such as lasers or telescopes, are manufactured using much more exacting techniques to acquire a much more precise mirror, but the general principles remain the same.