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August 30, 2009

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Mysteries of the old bronze mirror

IN ancient times, bronze mirrors with carvings on their back were one of the daily necessities. This bronze mirror displayed in Shanghai Museum (pictured below) is special, not only because of its beautiful carvings, but also its scientific mysteries.
The mirror named "Tou Guang Jing" was produced in the Western Han Dynasty (206 BC?AD 24).
The name means "mirror penetrable by light," however the mirror, which is made of bronze with patterns and characters carved at its back, is neither transparent nor translucent.
But when the sun shines on the mirror, the reflection is a bright circle with shadows of the carvings. It really looks like the sunshine has penetrated the mirror.
There are only four examples of this kind of mirror in Shanghai Museum among its hundreds of thousands of items collection of bronze mirrors. All four pieces were produced in the Han Dynasty. The reason is because, regrettably, the technique of producing the mirror was lost thousands of years ago.
The "penetrable" phenomenon has aroused the curiosity of ancient and modern scientists and attracted attention from abroad, being nicknamed the "magical mirror" by a Western scientist.
Only in recent decades has the secret been revealed: the surface of the mirror looks smooth, but actually there are extremely tiny lumps that match the carvings at the back.
Moreover, the carvings also make the structure form a residual stress inside the mirror that helps with the reflection. With the multiple factors that function in the mirror, when the sun shines, light reflected by the different lumps will form shadows that are exactly the same as the carvings on its back.
It is amazing to see in the mirror that Chinese people, living 2,000 years ago, had already discovered the theories of light reflection. The mirror is not only an artistic work, but also more importantly, it is an evidence of brilliant Chinese ancient scientific techniques and an example of the early ages' development of skills.
When the late Chinese premier Zhou Enlai visited Shanghai Museum in 1969, the mirror was selected as a national treasure to be highlighted to Zhou.


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