Re: "one-way" mirrors?

["Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU>]

On Thu, 22 Apr 1999, Jennifer M Seckinger wrote:

> Can someone please help me out on the issue of "one-way" mirrors?  One of
> my high school students has asked how the mirrors work when they allow
> people on one side to see a reflection, but allow people on the other
> side to see "through".  I'm almost positive that I remember talking about
> this in an undergrad class, but I don't remember maybe details.

These mirrors are misnamed. The secret isn't in the mirror but in its
method of use. The mirror has a semi-transparent metallic coating on one
side, so it transmits a small fraction of light, and reflects a lot. It is
mounted in a wall separating two rooms. I had an office once with a large
one-way mirror separating it from an adjacent room. It had once been used
to observe young children's behavior without them knowing they were being
watched, for educational research. The main room (with the children) was
brightly lighted, but the lighting should be arranged so that no direct
light falls on the mirror. The adjoining room (with the researcher/spies)
had black walls and no lights. The researchers wore dark clothing. Thus
the children could not see them unless they got close to the mirror and
looked very carefully. But if someone were to turn on a light in the
darkened room, the game would be up. Or, if someone in the lighted room
shone a bright light into the mirror.

However, the large partially-silvered mirrors are rather thick, so the
silvered (or maybe aluminized) surface should be facing the brightest
room, just as a regular mirror would, so one doesn't see double images.

The lighting arrangement could have easily been reversed, for the mirror
works both ways. It is the lighting arrangement and the fact that the
mirror transmits such a small fraction of the light, that makes the scheme
work.

Partially-silvered mirrors in small size can be bought from Edmund
Scientific for reasonable cost, and are good to have for experiments.

        -- Donald

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Donald E. Simanek
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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