Mirror: optical device, commonly made of glass, with a smooth, polished surface that forms images by the reflection of rays of light.

Mirrors made of brass are mentioned in the Bible, and mirrors of bronze were in common use among the ancient Egyptians, Greeks, and Romans. Polished silver was also used by the Greeks and Romans to produce reflections. Crude forms of glass mirrors were first made in Venice in 1300. By the end of the 17th century mirrors were made in Britain and the manufacture of mirrors developed subsequently worlwide.

The original method of making glass mirrors consisted of backing a sheet of glass with an amalgam of mercury and tin. The surface was overlaid with sheets of tinfoil that were rubbed down smooth and covered with mercury. A woolen cloth was held firmly over the surface by means of iron weights for about a day. The glass was then inclined and the excess mercury drained away, leaving a lustrous inner surface.

The first attempt to back the glass with a solution of silver was made by the German chemist Justus von Liebig in 1836; various methods have been developed since then that depend on the chemical reduction of a silver salt to metallic silver.

In the manufacture of mirrors today, in cases where this principle is utilized, the plate glass is cut to size, and all blemishes are removed by polishing with rouge. The glass is scrubbed and flushed with a reducing solution such as stannous chloride before silver is applied, and the glass is then placed on a hollow, cast-iron tabletop, covered with felt, and kept warm by steam. A solution of silver nitrate is poured on the glass and left undisturbed for about 1 hour. The silver nitrate is reduced to a metallic silver and a lustrous deposit of silver gradually forms. The deposit is dried, coated with shellac, and painted.

In other methods of mirror production, the silver solution is added with a reducing agent, such as formaldehyde of glucose.

Silvering chemicals are often applied in spray form.

Special mirrors are sometimes coated with the metal in the form of vapor obtained by vaporizing silver electrically in a vacuum. Large mirrors are often coated with aluminum in the same way.

In addition to their general household use, mirrors are used in scientific apparatus, for example, as important components in microscopes and telescopes.