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How does a one-way mirror allow you to see through in only one direction?

The windows of Boston's John Hancock Building act as mirrors in daylight. The windows of Boston's John Hancock Building act as mirrors in daylight. (TOM LANDERS/GLOBE STAFF file)

The term "one-way" mirror is a bit of a misnomer since a one-way mirror actually transmits light equally well in either direction. It only acts the way you want it to under special conditions of illumination.

Normal mirrors are made of glass with a metal coating so thick that no light gets through at all - that is to say, all the light that hits it is reflected. (Actually, a bit always gets absorbed, but we'll neglect that for the rest of this piece since it's not important to the overall explanation).

If the metal coating is made thin enough, then one can get a mirror which reflects some fraction of the incident light and allows some light to go through. This is sometimes called a semitransparent mirror, and is what one wants to use as a "one-way" mirror.

The mirror has to be between two places, one that is brightly lit and the other dark. If you look from the dark side, whatever fraction of light from the bright side that makes it through the mirror will be bright compared to the darkness and so you'll be able to see what's on the other side.

If you're on the bright side, you'll see a combination of the little bit of light from the dark side that gets through the mirror plus lots of light from your side reflected from the mirror. This reflected light will swamp out what comes through from the dark side and you won't be able to see through.

In addition to uses in police and psychiatric work, one-way mirrors also find applications in big office buildings. Using partially transparent mirrors as windows means that in the daytime (when the light outside is brighter than the light inside), the building looks from the outside like it's covered in mirrors, and you can't see inside, while people inside can see out.

At night, however, when it's dark outside, you can easily see what's going on inside a lit office, while for someone in that office the window looks like a mirror.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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