Mirror - it is the brightness of the
mirror surface that makes it reflect light.
When light falls on a surface some of the
light may be reflected or thrown back, some absorbed and some allowed to pass
through. In a mirror the surface is made so bright that as much light as
possible is reflected and as little as possible absorbed.
The earliest
mirrors consisted of thin discs of metal, generally bronze, slightly convex and
polished on one side.
The method
of making mirrors by backing glass with thin sheets of metal was known in the
Middle Ages, and a guild of glass-mirror makers existed in Nurnberg, Germany,
in 1373. The commercial manufacture of mirrors was developed in 16th Century
Venice. Coated mirrors were made from blown cylinders of glass which were slit,
flattened on a stone, polished, and their backs silvered by an amalgam of tin
and mercury.
These
mirrors had a high reflecting power, but a considerable improvement came in
France in 1691 when the art of making plate glass was introduced. The chemical
process of coating a glass surface with metallic silver was discovered by Baron
Justus von Liebig of Germany in 1835. Mirror
surfaces are used inside lighthouses, lightships and searchlights, where it is
necessary to produce a high degree of reflection in order to throw the beam of
the light over a distance of several miles. Even a hand flashlight has a
slightly mirrored surface behind the bulb.
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