madsci.org
Dated: Tue Jun 22 17:18:39 1999
Posted BY: Steve Guch, Post-doc/Fellow, Physics (Electro-Optics/Lasers), Inc., Laser Systems Division
Area of science: Physics
ID: 928703198.Ph
Message:
I think you mean "one-way" glass - i.e., a glass surface that you can see through in the other direction - rather than
"two-way" glass.
"One-way" glass is coated with a thin metal layer - usually aluminum that reflects most of the light that reaches it from
either side. The mirrored glass is located between one room that is relatively dark and the another that is brightly
illuminated.
When you are sitting in the dark room, there's not much light originating inside your room, because you've turned the
lights way down. When you're sitting in the bright room, there's lots of lights inside your room. Maybe 100 times the
amount of light that's in the dark room. An easy way to think of this is that the bright room may have a few 100 watt bulbs
in them, while the dark room will have a very dim 1 or 2 watt night light in it - just enough so you don't trip and kill yourself.
When you're in the dark room, the reflection of the small amount of light in your room produces very dim images after
being reflected from the mirror back in your direction. The light from the bright room is attenuated considerable when it
goes through the mirror, but is still bright enough to overwhelm the small amount of light that originates in the dark room
and is reflected back.
When you're sitting in the light room, the reflection of the large amount of light in your room produces a pretty bright image
after being reflected from the mirror back in your direction. The light from the dark room is attenuated considerably when
it goes through the mirror, so that the light reaching you is exceptionally weak - too weak to really be seen as an image
because the bright - room reflection is so strong.
Let's look at the mathematically, assuming that the mirror reflects 95% of the light that hits it back into the room it came
from:
In the dark room, you get the following light from the two sides: From the dark side - 1 watt light X object reflectivity X 95%
mirror reflectivity = ..95 X object reflectivity
From the light side - 200 watts light X object reflectivity X 5% mirror transmission = 10 X object reflectivity. So the image of
objects in the light side area is 10 times as bright as the image of the objects in the dark area and can readily be seen -
although you'll probably see a spooky ghost image of activities in the dark area.
In the light room, the get the following light from the two sides: From the dark side - 1 watt light X object reflectivity X 5%
mirror transmission = .05 X object reflectivity. From the light side - 200 watts light X object reflectivity X 95% mirror
reflectivity = 190 X object reflectivity. So the image of objects in the light side area is 3800 times as bright as the image of
objects in the dark side area. The image of objects in the dark side area is so dim, by contrast to the one from the objects
in the bright side area, that you just don't notice that it's there.
If you vary the reflectivity of the mirror - by changing the thickness of the very thin metal layer - you change the magnitude
of the two effects pretty dramatically. If you make the mirror 99% reflective, it will be much harder for the viewers in the
light room to see through the mirror - essentially impossible under any circumstances. But you'll have to reduce the light
in the dark room to ensure that the image from the bright room is visible. If you make the mirror 80% reflective, you
probably run the risk of having the viewers on the light side from seeing the folks on the dark side.
As you can see, the real trick of "one-way" mirrors is to be sure that the folks on one side are in the darkness and those
of the other are in brightness. What they really do is to provide enough light transmitted through so that the people on the
dark side can see the image of the scene on the bright side, while the reflection of the light from the objects on the light
side is sufficiently high and transmission of the light from the objects on the dark side is sufficiently small that there is
insufficient contrast for the eye to see the items on the dark side.
As far as how you might be able to build one, about the only thing you have to do is find somebody to sell you some
mirror glass to put between two rooms in which the lighting level is dramatically different. Alternatively, it might be less
expensive to get pieces of reflective film of th sort that are used to "tint" car or home windows to keep the sun loading
inside the vehicle or structure down. The reflective films generally look a little wavy, but they will work. [Note: Be sure to
get the film that has a reflective metallic coating on it, not the kind that has an absorptive coating - the absorptive kind will
produce a weakened one-way effect, but will not fool anybody on the bright side because it won't look like a mirror... . It'll
just look like a piece of grey plastic.]
