Magic Mirror Isn’t Transparent Metal

One of the Star Trek movies has a McGuffin called “transparent aluminum.” While magic mirrors aren’t really transparent, it appears that way to a casual observer. If you haven’t seen one of these, they are polished metal mirrors with a pattern embossed on the back. When you shine a point source of light on the mirror, however, the reflection matches what is on the back of the mirror. Is it transparent? No, and the video by [Steve Mould] below explains what’s really going on.

The reality is that very subtle variations of the surface produce the image. You need some understanding of optics and calculus to fully understand what’s going on.

The real mystery, though, is how these parts are made. There is some speculation about the thickness of the embossing, but keep in mind that the surface variations are a fraction of a micron. Even more interesting is that not all mirrors reflect the image on the back, which makes you think the image on the back is just a subterfuge and not necessarily part of the fabrication process.

We figure if anyone can figure out how to make these, it is our readers. We’ll entertain any speculation other than using a 555 or an Arduino. We wonder if you could cast these out of epoxy. We’ll admit optics aren’t our strong suit, but we do dig into it occasionally. There’s always a use for a fresnel lens.

20 thoughts on “Magic Mirror Isn’t Transparent Metal

  1. It has been Known for years that even if you grind the serial numbers off of a piece of metal (engine block, gun, etc.) the numbers can still be recovered through stresses made in the base metal by the stamping process.

  2. Maybe the image is cast onto the mirror side before being ground and polished off. Annealing might then be just enough to bring back the image as the stresses from the casting reveal themselves.

  3. Press the design on the back of the mirror while polishing it; a slight deformation, caused by an increase or decrease in the pressure while grinding its slight curve.

  4. doi:10.1088/0143-0807/27/1/012
    The modern day authority on magic mirrors is Dr. Barry, who gives the physical description of what is occuring. While cast metals have unique behaviours, like surfaces being put into tension or compression during cooling, Dr. Barry mentions that this can also be observed in glass. So it would make sense that polymers could be cast and polished in a similar manner. There is also the ability seen in polymers to hot transfer surface distortions/patterns which results in holographic like images due to constructive and destructive interference at the surface.

  5. The “transparent aluminum” of Star Trek isn’t a McGuffin at all. It’s a reality that predates historic civilization. You and I know transparent aluminum very well. The chemists among us know it as aluminum oxide. The common folk know it as rubies and sapphires. And, yes, there are white Sapphires and very pale rubies.

    1. It’s not really a MacGuffin, but not because sapphires are real. A MacGuffin doesn’t have to be fictional, it just has to be an object that drives the plot purely by it’s existence and not by any particular feature it has (i.e. the plot could be identical if you replaced the thing with something completely different, as nothing actually cares what it is.)

      In Star Trek IV, Scotty and Bones need to pay for something and trade information about a future technology for it. It’s not really any different to Kirk pawning his antique glasses. Both things play a very minor role in the movie, and *that* is why they aren’t MacGuffins. If anything, the MacGuffin is the whale!

      On a separate point, what Scotty calls “Transparent Aluminum” is not aluminium oxide. It’s a fictional material with similar structural properties to metallic aluminium while also being optically clear. If it was sapphire then 1) Scotty would have called it sapphire and 2) it would have been worthless as payment for the items they were after…

  6. Considering the shallow depth required, why couldn’t you simply print the reverse image on a polished surface with something as simple as shellac/enamel, then once dry, polish the surface evenly, then remove the shellac? These lacquers can easily be applied in micron thicknesses.

    Clay-tempering could also work by hardening some areas making them lose less material during polishing, but probably not at this small size and resolution. Maybe etch an image with a laser to temper near the surface then polish out….but that wouldn’t explain how it was historically done.

    Just embossing the pattern should provide this hardening effect at the desired resolution. So if the mirror surface (not rear) is first embossed, then ground down, you’d probably get this image without any additional effort.

    Makes sense that historically an object which predates electric light depends on a point source of light to work. So I think the curve of the mirror isn’t needed to function.

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