The oldest manufactured mirrors were polished volcanic glass, some 8,000 years old
Before glass and silver, people looked at themselves in obsidian, a natural volcanic glass polished to a shine. Examples from Çatalhöyük in Anatolia date to around 6000 BCE. For most of history after that, a mirror meant a polished disc of metal, and a good one was a luxury.
Any surface can act as a mirror if it is flat enough and its roughness is finer than the wavelength of the light hitting it. Then incoming waves bounce off with their shape intact, each ray leaving at the same angle it arrived, and a lens such as the eye can focus them into an image that seems to sit behind the glass. Rougher surfaces, like white paint, scatter the waves in all directions instead. The first mirrors were almost certainly still pools and shiny stones.
Polished copper mirrors were made in Mesopotamia from about 4000 BCE and in Egypt from about 3000 BCE, and stone mirrors in Central and South America from around 2000 BCE. By the Bronze Age most cultures used metal discs; the people of Kerma in Nubia were skilled mirror makers, and bronze kilns survive inside their temple. China produced bronze mirrors from around 2000 BC. Metal remained the norm through Greek and Roman times and the European Middle Ages.
A copper-tin alloy called speculum metal, which may have originated in China and India, reflected especially well. Isaac Newton used it in his reflecting telescope of 1668, and the Great Melbourne Telescope in Australia still relied on it in 1869. Like mirrors of precious metal, it was hard to make and mainly owned by the rich. Modern mirrors instead put a thin coat of silver or aluminium on glass, prized for its hard, naturally smooth surface.
The idea extends beyond light. Walls and rock faces act as acoustic mirrors, producing echoes that architects of concert halls and studios must tame, and parabolic sound reflectors are used in microphones and sonar. Physicists have even built atomic mirrors that reflect matter waves. Mirrors also steer light inside lasers, periscopes and cameras, and folklore warns that breaking one brings seven years of bad luck.
Source: Mirror
