How is the famous ancient cultural relic "Guang Guang Sword" in China recorded and explained in Shen Kuo's Bi Tan?

Shen Kuo is also an epitaph and archaeologist. In the 19th volume of Bi Tan, he gave a detailed account of the artifacts he collected or saw. What is particularly worth mentioning is the record and explanation of the famous ancient cultural relic "Bald Sword" in China. The production of "transparent mirror" is a special mirror-making technology created by the working people in ancient China by applying the principles of physics, which has a very high technical level and artistic level. According to Bitan, this mirror is made of copper, with raised patterns and characters on the back and polished on the front. With the mirror facing the sun, the pattern on the back of the mirror can be clearly reflected on the wall. For this phenomenon, Shen Kuo also made a more scientific explanation. He said: "When casting a mirror, in the process of cooling, the thin place (where there is no pattern) is cold first, and the thick place (where there is raised pattern) is cold later. The density of copper in the first cold place is different from that in the second cold place, so the ability to reflect light is different. So a polished mirror can reflect the pattern to the wall. "