Lore has it that some magical illusions fool only the eyes. They do not appear in a mirror.

Is there a way to harness this? Could a hunter of wizards and magical creatures build a set of mirrored goggles with a double-mirror system that filters their sight through two mirrors? This would allow them to see through illusions. I am not an engineer, so I'm not sure if that would be possible/feasible to wear into a hunting and tracking situation.

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    $\begingroup$ TBH I'm pretty sure I could have something like this built by dinnertime.. $\endgroup$ – Morris The Cat Feb 10 '20 at 18:43
  • $\begingroup$ See "how to slay the Gorgon Medusa" Been done several thousand years ago $\endgroup$ – Carl Witthoft Feb 10 '20 at 20:38
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    $\begingroup$ It depends on the nature of the thing they're looking at. I can think of one, possibly two, fictional visual effects (one fatal, the other debilitating) where the author allowed a single mirror to prevent the effect, but a double mirror would not. The trick of course was to make a magical creature with its head on backwards, using a single mirror to see what was in "front" of it, to safely hunt the source of the effects, $\endgroup$ – Starfish Prime Feb 10 '20 at 22:09
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    $\begingroup$ Vampire: put down that stupid thing on your head already, are we gonna fight or not! $\endgroup$ – user6760 Feb 11 '20 at 1:19

Sure. It's just a periscope. You'd probably make it shorter both for wearability and durability, but if you have good mirrors, it'd work fine.

(The major problem would be if it were a medieval setting or something, where good, silvered mirrors would be hard to come by.)

  • $\begingroup$ one advantage of periscope is that you can make it stick sideways from the eye, allowing wearer to peek around corners, and enhancing depth perception. Given the setting, can have periscopes go sideways and up, resembling horns attached to a helmet. $\endgroup$ – Bald Bear Feb 10 '20 at 18:43
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    $\begingroup$ Silvered mirrors are a very late invention (19th century, Justus von Liebing). But since the late Middle Ages (roughly from the 13th century onwards) they had perfectly serviceable lead-backed glass mirrors; fabulously expensive if large, only moderately expensive if small. Mirrors were extensively used by the Renaissance painters, for example, as aids in drawing their realistic pictures etc. $\endgroup$ – AlexP Feb 10 '20 at 20:16
  • $\begingroup$ A solid glass periscope prism is possible without silver but would require good clear glass and optical flat polishing at accurate face angles. $\endgroup$ – KalleMP Feb 11 '20 at 7:58

You ask, science answers!

Enjoy the wonder of the pentaprism


A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness) as an ordinary right-angle prism or mirror would.

The reflections inside the prism are not caused by total internal reflection, since the beams are incident at an angle less than the critical angle (the minimum angle for total internal reflection). Instead, the two faces are coated to provide mirror surfaces. The two opposite transmitting faces are often coated with an antireflection coating to reduce spurious reflections. The fifth face of the prism is not used optically but truncates what would otherwise be an awkward angle joining the two mirrored faces.

  • $\begingroup$ They use these in high-magnification binoculars don't they? $\endgroup$ – Morris The Cat Feb 10 '20 at 18:43
  • $\begingroup$ A little bit more complex than a simple pentaprism, but the idea is right. See binocular prisms $\endgroup$ – Alexander Feb 10 '20 at 19:14
  • $\begingroup$ The pentaprism still needs a mirror to look forward and then the image is inverted if you do not include a lens in front. Not a suitable answer in this case unless you decided to use two and then mirrors would make much more sense. $\endgroup$ – KalleMP Feb 11 '20 at 7:57

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