Date of creation: 2nd century BC.

Work? Liu An, King of Huainan.

Class? Other important documents related to physics and chemistry.

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"Huainan Wanbi Book" records that the ice lens makes a fire: "Cut the ice to make it round, lift it to the sun, and bear its shadow with Ai, then the fire will be born." To make a fire, it is customary to use heat, but here it is ice. This record is not whimsical. Zheng, an optician in Qing dynasty, proved through simulation experiments that it is feasible to use ice lens to make fire. Moreover, this article clearly mentions the focusing property of the lens, which is also valuable.

In the late Qing Dynasty, Zheng had done a simulation experiment. He used a tin pot with a slightly concave bottom, filled with hot water, ironed it on the cut ice to get a crystal clear ice lens, and then used it to take fire from the sun, which succeeded. He summed up the essentials of using ice lens to make fire, saying, "However, if the sun is bright, the ice is bright and bright, with a slightly shallow convex shape (about three inches in diameter and about two feet in outer limit), it must be slowly and steadily obtained." Zheng's experience is reasonable.

special technology

The light collecting ability of a lens is the square of the ratio of its aperture to focal length (relative aperture), so a lens with a large aperture is beneficial to improve the light collecting ability. On the other hand, shallow convexity leads to large focal length, which is not conducive to light collection. In this regard, Zheng went on to explain: "Gaihuo was born when the sun was hot. Although it is not a mirror, ice has cold air, which can reduce the heat of the sun. Therefore, it is necessary to have a large convex and shallow diameter, so that the air conditioning is far away and powerful. " It turns out that this is because the ice is cold, so the focal length should be slightly larger to reduce the cold effect of the ice lens and make the fire fighting easy to succeed.

PS: Many lenses in ancient China were polished with crystal, and many of these lenses were unearthed. It's even 3,000 years old.