What is the light transmission principle of bronze mirrors in the Western Han Dynasty?

Principle: In the casting process, due to the different thickness of the concave-convex part of the mirror back, casting stress (physical phenomenon) is generated through solidification contraction, and then compressive stress is generated through grinding, thus forming elastic deformation in physical properties.

When polished to a certain extent, this elastic deformation is superimposed, resulting in a corresponding curvature between the mirror surface and the mirror back pattern, thus producing this light transmission effect. A large number of ancient transparent mirrors have been unearthed in Shanghai and Henan.

Extended data:

For more than 1000 years, the magical phenomenon of transparent mirror has attracted many scholars at all times and at home and abroad to study it. From Shen Kuo in the Song Dynasty to Zheng in the Qing Dynasty, from China to the other side of the ocean, countless scholars have racked their brains to try to uncover the mystery of ancient mirrors that have plagued the world for thousands of years.

Shen Kuo and Zheng found that the transparent mirror was thick with inscriptions and patterns on it, but thin without inscriptions. The edge of the transparent mirror has a wide and thick mirror ring. When casting bronze mirrors, the cooling speed is fast due to the thin mirror body, while the cooling speed is slow due to the wide mirror ring.

When the mirror body has been formed, the mirror ring is still cooling and shrinking, which tightens the mirror body and forms casting residual stress. However, when the mirror body is deformed, the mirror bottom is largely arched towards the mirror surface, while the mirror ring is slightly arched, so the mirror surface is uneven.

Although this difference is unrecognizable to the naked eye, it is enough to make the distribution of reflected light different. When a beam of parallel light is projected on the mirror, the patterned place is flat, and the reflected light is concentrated, while the non-patterned place is convex and the reflected light is divergent, so the light reflected on the wall from the patterned place is bright, while the non-patterned place is dark, and the wall presents the same pattern as the mirror back.

Another feature of the transparent mirror in the Western Han Dynasty is that its patterns are distributed in the circumferential direction. Only in this way can significant circumferential casting residual stress be generated and the mirror surface can form regular concavities and convexities. Therefore, after studying in Qing Dynasty, Zheng came to the conclusion that "casting causes grinding deformation".

References:

Baidu encyclopedia-translucent mirror