How Do "Chinese Magic Mirrors" Work?

Ever seen what's known as a Chinese Magic Mirror? These mirrors, made of polished bronze and dating back to the Han Dynasty, have a design on the back. When you shine a light on the front, the front of the mirror projects the image of that design onto a screen. Cool! But how?

You can find these cute little items in magic shops, or in souvenir shops. The first recorded instance of them seems to date back to around 200 BC in China; sometimes they're called "Japanese Magic Mirrors," as many early versions of them have also been unearthed in Japan. Here's what happens when people shine a light on the front:

For a long time, people wondered how the transmission of the design from the back to the front worked. There were clues; the design disappeared when anyone pared the edges of the mirrors off, leading some people to think there were some kind of difference in the microstructures of the metal in the thick or thin parts. Meanwhile, mirror-makers found that if they made a normal mirror was very thin, it acquired the properties of a magic mirror. If it were microstructures in the metal, they would have to also appear in glass.

The microstructure idea was dismissed when teams of scientists began coating the surface of the mirrors with ultrathin layers gold, silver, or aluminum. If the pattern were the result of microstructures, the design would have disappeared, but the mirrors projected the design as well as they always did.

The answer turned out to be relatively mundane. Although the surfaces of the mirrors are polished, there are slight convex and concave curves caused by the design. The convex (outgoing) curves scatter light, and make their areas of the reflection darker. The concave ones focus light and make their areas of the reflection lighter. The mirrors are cast (poured as liquid into a mold), and the thicker parts and thinner parts cool at different rates. Because metal slightly contracts as it cools, the different rates of cooling "stress" or deform the metal a bit.

Thinner areas are also more flexible than thicker ones, and so the polishing process, which should smooth metal until its uniform, exaggerates the slight differences in thickness. Although we can't see the slight pattern on the mirror's surface, photons can pick it out very clearly, so when they are allowed to bounce off the curves of the mirror, a pattern emerges.

[Via Recent Advances in Understanding the Mystery of the Ancient Chinese Magic Mirrors]