Two-way mirrors - How do they work?

[Richard M. Flood]

I hope this is the best group for this, so here goes. How are Two-way
mirrors made and how do they work? I have allways been curious about
that. I also remeber hearing a story that there was a way to tell the
diffrence between a real mirror and a two-way mirror. Any ideas on
this? I've been curious about this since reaching the optics chapters
in physics.

-Rich

[Harvey Rutt]

>-Rich

The short answer is that in real life there is no such thing as the
two way mirror of spy stories etc - ie no mirror that passses light in
one direction & not in the other.

Mirrors which *appear* to do this are partly reflecting with a small
amount of transmission.
One side is brightly lit, the other dimly.
If you are on the bright side you see your own reflection because it
is much brighter than the weak transmitted light from the 'dim' side.
On the dim side the transmitted light is stronger than your own
reflection so you see through.

The are tricks using Faraday rotation & polarisers or quarter wave
plates & polarisers which do transmit in one direction, but for one
colour only & not as a big mirror.

Harvey Rutt
ps how to tell?
Tricky, unless you can measure the reflectivity.
If you go up to it & shield you eyes so the light on your side is
thoroughly blocked you might well then see through, unless the dim
side is in total darkness. In that case do the same thing, shine a
laser pointer on it & see if there is a displaced red spot visible
thro' the mirror; defeated if the dim space has black walls!