Makyoh

The Makyoh (Japanese for "magic mirror") is an ancient art that can be traced back to the Chinese Han Dynasty (206 BC — 24 AD). Makyoh were made of copper alloy, usually with an intricate pattern carved or cast on the back and the front polished to a mirror finish. The front looks like a smooth reflecting surface, but when sunlight or other bright light is reflected onto a wall a glowing pattern emerges. Usually the image seen would be the same as the image on the back of the mirror, often an image of the Buddha or other focus for meditation. The art later moved to Japan (especially Kyoto), and after missionaries brought Christianity into Japan in the mid 1500s many mirrors were made with secret images of the Holy Cross or of Christ. Because Christianity was punished at the time, many Christians wore such magic mirror as a secret sign of their faith.

The "magic" of a makyoh comes from fact that, while the mirror's surface appears smooth, the pattern to be projected is held latent within the metal in differential hardening after annealing, and the surface when polished is differentially abraded. This causes parallel light beams (such as from the sun or a distant light source) to reflect at slightly different angles and form the desired image.

A similar principle is used in a technique known as makyoh topography, which is used to determine the flatness of manufactured materials such as semiconductor wafers.