<P> Glass was a desirable material for mirrors . Because the surface of glass is naturally smooth, it produces reflections with very little blur . In addition, glass is very hard and scratch - resistant . However, glass by itself has little reflectivity, so people began coating it with metals to increase the reflectivity . Metal - coated glass mirrors are said by the Roman scholar Pliny the Elder to have been invented in Sidon (modern - day Lebanon) in the first century A.D., although no archeological evidence of them date from before the third century . According to Pliny, the people of Sidon developed a technique for creating crude mirrors by coating blown glass with molten lead . Glass mirrors backed with gold leaf are mentioned by Pliny in his Natural History, written in about 77 A.D. Because there were few ways to make a smooth piece of glass with a uniform thickness, these ancient glass - mirrors were made by blowing a glass bubble, and then cutting off a small, circular section, producing mirrors that were either concave or convex . These circular mirrors were typically small, from only a fraction of an inch to as much as eight inches in diameter . These small mirrors produced distorted images, yet were prized objects of high value . These ancient glass mirrors were very thin, thus very fragile, because the glass needed to be extremely thin to prevent cracking when coated with a hot, molten metal . Due to the poor quality, high cost, and small size of these ancient glass mirrors, solid metal - mirrors primarily of steel were usually preferred until the late nineteenth century . </P> <P> Parabolic mirrors were described and studied in classical antiquity by the mathematician Diocles in his work On Burning Mirrors . Ptolemy conducted a number of experiments with curved polished iron mirrors, and discussed plane, convex spherical, and concave spherical mirrors in his Optics . Parabolic mirrors were also described by the physicist Ibn Sahl in the tenth century, and Ibn al - Haytham discussed concave and convex mirrors in both cylindrical and spherical geometries, carried out a number of experiments with mirrors, and solved the problem of finding the point on a convex mirror at which a ray coming from one point is reflected to another point . By the 11th century, glass mirrors were being produced in Moorish Spain . </P> <P> In China, people began making mirrors by coating metallic objects with silver - mercury amalgams as early as 500 A.D. This was accomplished by coating the mirror with the amalgam, and then heating it until the mercury boiled away, leaving only the silver behind . </P> <P> The problems of making metal - coated, glass mirrors was due to the difficulties in making glass that was very clear, as most ancient glass was tinted green with iron . This was overcome when people began mixing soda, limestone, potash, manganese, and fern ashes with the glass . There was also no way for the ancients to make flat panes of glass with uniform thicknesses . The earliest methods for producing glass panes began in France, when people began blowing glass bubbles, and then spinning them rapidly to flatten them out into plates from which pieces could be cut . However, these pieces were still not uniform in thickness, so produced distorted images as well . A better method was to blow a cylinder of glass, cut off the ends, slice it down the center, and unroll it onto a flat hearth . This method produced the first mirror - quality glass panes, but it was very difficult and resulted in a lot of breakage . Even windows were primarily made of oiled paper or stained glass, until the mid-nineteenth century, due to the high cost of making clear, flat panes of glass . </P>

When do we use the different types of mirror