Also known as two-way glass, a two-way mirror is glass that is reflective on one side and clear on the other, giving the appearance of a mirror to those who see the reflection but allowing people on the clear side to see through, as if at a window. Most people are familiar with the concept of a two-way mirror/two-way glass from Hollywood’s depiction of police dramas and the questioning room with two-way glass for observation of interrogations, but this isn’t their only use. Many ballet studios use them so that parents can watch their children practice during lessons.

Two-way mirrored glass is made the same way traditional mirrors are made, with a thin coating of metal behind the glass; however, two-way mirrored glass only has half the amount of metal backing that traditional mirrors do. One coat of metal causes most of the light to reflect back toward the source but does prevent some of it from passing through, which allows occupants on the other side to see through. The metal coating does, however, darken what can be seen from the non-reflective side, giving the impression of looking through a tinted window.

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