Re: How does a two-way mirror work? How could I make one?

I think you mean "one-way" glass – i.e., a glass surface that you can see 
through from one direction and can’t 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 another that is brightly 
illuminated.  

When you are sitting in the dark room, there’s not much light originating 
inside your room, because you’rve turned the lights way down .  When you’re 
sitting in the bright room, there’s lots of light 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 considerably 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 imge because the bright-
room reflection is so strong.

Let’s look at this 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 dar 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 can 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 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 to 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 the sort that are used to 
"tint" car or home windows to keep the sunloading 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 absorbtive 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.]