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© Copyright 1999, Jim Loy
A two-way mirror is often called a "one-way mirror" by members of
the general public. The misconception is that such a mirror acts as a mirror
from one side, and acts as a window (letting light through) from the other
side. Actually, the two-way mirror is letting about half of the light through,
and reflecting the other half of the light, from both sides. it is also called
a half-silvered surface, as just enough reflecting metal film (usually aluminum
as far as I can determine) is deposited on the glass, so that about half of the
light is reflected.
So, why does a two-way mirror seem to behave like the two sides are different? It behaves this way when one side is in the dark. Then almost no light goes from the dark side to the light side, and almost no light is reflected back from the dark side to the dark side. Most of the light comes from the bright side. Plenty of light travels through the mirror, and plenty of light is reflected back. To people on both sides of the mirror, the light from the bright side overwhelms the light from the dark side. So, people on the bright side see a mirror, and people on the dark side see a window. See the above diagram.
Robin Williams told a joke about policemen in the South having mirrors on the inside of their glasses. Good joke, but such glasses are two-way mirrors, and are shaped so that your eyes are always in the dark.
See two-waymirrors.com/.
