Magic Mirrors - Page 2

Not all Magic Mirrors are "Magic"

Magic Mirrors of this type, with a handle, can sometimes be seen in antique shops and at specialised auctions. For the collector they are beautiful objects with elaborate, finely worked relief patterns on one face. Unless the mirror has been well protected the "shiny" face will usually be dull and damaged and unlikely to work, even as a normal mirror. The mirror surface needs to be in very good condition and should also be slightly convex, although this is not always obvious. Even so the mirror may not be "magic". It seems that some Japanese mirrors are magic and some are not, although I understand that most Chinese mirrors are magic.

A bit of optics

If you did any optics at school you may have been taught that for a mirror to produce a "real image" (that is, one that can be projected onto a screen) the mirror must be concave. Large astronomical telescopes use huge concave mirrors. For a lens to produce a real image it must be convex; fatter in the middle than at the edges. Concave lenses and convex mirrors form Virtual Images, which can be seen using another lens or directly with our eyes, but they cannot form real images on a screen. So there seems to be a bit of a paradox. The magic mirror is slightly convex but it certainly produces a real image on a screen.

Historical

The earliest British reference I can find so far is in the Philosophical Magazine 1832:- "Account of a curious Chinese Mirror, which reflects from it's polished Face the Figures embossed upon it's Back. By Sir D. Brewster K.H. LLD" (Sir David Brewster 1781-1868 was a Scottish physicist famous for the discovery of polarisation of light by reflection. When light is reflected from a non-metallic surface, such as glass, partial polarisation occurs. He invented the kaleidoscope, another delightful optical toy).

Brewster refers to an account he has just received from George Swinton of Calcutta, about a curious metallic mirror recently brought from China to Calcutta, which was "amusing the dilettante and perplexing the philosophers" of that city. Swinton suggests that the mirrors are made by stamping and that this produces a difference in the density in different parts of the metal, the light being reflected more or less strongly from parts that have been more or less compressed.

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