Mirrors have always been perceived as subtly magical. More particularly:
1. A mirrored face is uncannily lifelike and can easily be imagined as speaking autonomously; hence the talking magic mirror best known from the Snow White tale, a frequent fantasy prop which has often been spoofed – e.g. in Anthony Armstrong's stories. (Breaking a mirror is bad luck since one shatters one's own image, with obvious sympathetic-Magic implications.)
2. The ability to reflect, instantly, whatever may be happening close at hand extends by natural analogy to the display of scenes far off in space or time: hence mirrors can be Scrying devices, as with the Magic Mirror in Disney's Snow White and the Seven Dwarfs (1937). Many traditional magic mirrors operate thus: the God Vulcan's showed past, present and future; the Moon King's mirror in Lucian's True History revealed events anywhere on Earth, as did Merlin's in Edmund Spenser's The Faerie Queene (1590-1596). Cambuscan's mirror in Geoffrey Chaucer's "The Squire's Tale" warned of national ill-fortune and the falsity of women. The mirror of Galadriel in J R R Tolkien's The Lord of the Rings (1954-1955) is a reflecting pool which displays potential futures. Avram Davidson's The Phoenix and the Mirror (1966) invokes much learned Alchemy and Astrology for its preparation of a "virgin speculum" crafted in darkness, whose first fleeting image is a Portent.
3. A room's mirror reflection seems subtly awry owing to reversal, suggesting a different room in an Otherworld behind the glass, with the mirror now a potential Portal through which, for example, Alice can climb in Lewis Carroll's Through the Looking-glass (1871). Combining 2 and 3 implies portals to anywhere and anywhen, as in Stephen R Donaldson's Mordant's Need, where in accordance with a Rationalized-Fantasy law the precise curvature of each mirror surface "tunes" it to a specific destination. Father Inire's mirrors and related numinous mirrors in Gene Wolfe's The Book of the New Sun (1980-1983) allow both distant viewing and travel, and also allude to Jorge Luis Borges's notes on fauna inhabiting mirrors in The Book of Imaginary Beings (1967). The mirror of the inset story in Phantastes (1858) by George MacDonald shows the expected room, but with a new inhabitant compelled there by Bondage.
4. When mirror faces mirror the infinite corridor of repeated reflections is eerie in itself. Fritz Leiber's "Midnight in the Mirror World" (1964) has a suicide's Ghost visible in the depths, moving closer by one reflection each night. The egregious savant de Selby in Flann O'Brien's The Third Policeman (1967) studies the sequence's far-off and therefore older images through a telescope, ultimately seeing himself aged 12. The Villain of Terry Pratchett's Witches Abroad (1991) uses facing mirrors for her magic, feeding on her multiplied self but being spread thin along the line of images in a double metaphor of hubris and Thinning: "Mirrors give plenty, but they take away lots."
5. Mirrors can also work against magic. They may reflect Spells, or usefully not reflect the deadly component of Medusa's gaze. They offer images truer than human Perception, and do not show unrealities like Vampires or Ghosts. A disguising Illusion or Glamour will probably fail when viewed in the mirror. [DRL]