Welcome to You Ask Andy

John Spears, age 11, of Hartsville, S.C., for his question:

WHO INVENTED THE MIRROR?

A mirror is any smooth surface which reflects more light rays than it absorbs. The more highly polished a mirror is, the more it will reflect. Even a highly polished sheet of clear glass will serve as a mirror, although most rays of light pass through.

Good mirrors today are made of polished glass with a silvery back surface. 'The condition of being opaque and the smoothness of the surface determine the quality of the reflection the mirror will produce.

Earliest mirrors were used by the Etruscans, Greeks and Romans. We do not know who actually invented the mirror. Metal discs, generally made of bronze, were highly polished and worked quite well during these ancient times.

Coating glass with tin was known in the Middle Ages. There was a guild of glass mirror makers in Germany as early as 1373, although the first commercial production didn't start until the 1500s when the industry developed in Venice, Italy.

Great improvement came to the mirror business in France in 1691 when the art of making plate glass was introduced. And then in 1835 a man by the name of Justus Von Liebig came up with the formula for coating a glass surface with metallic silver. The process was, and still is, called silvering.

In a mirror, the angle between a light ray striking a mirror and the normal (perpendicular) to the mirror is the angle of incidence. The angle between the normal and the ray reflecting off the mirror is the angle of reflection. These two angles are always equal.

There are three kinds of mirrors: the flat, concave and convex. A concave mirror is o'ne whose surface is like the inside of a hollow ball. It will reflect a larger or smaller image, depending on the object's distance from it. If the object is further away, the image will appear upside down.Concave mirrors are used in reflecting telescopes.

A convex mirror always shows an image smaller than the object.

Construction of a large astronomical mirror requires work of the highest precision. The front mirror surface must not deviate from the mathematically perfect paraboloid of revolution by more than 1 millionth of an inch. The final polishing process may take from three to five years. The glass used is a special type of Pyrex which expands only slightly when it is heated.

The world's largest reflecting telescope is 200 inches wide. It is located at Mount Palomar in California.

 

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