Y: Today on A Moment...

D: Psst! Yael. Do you see that mirror on the wall? I think it's a two-way mirror, and that      management is spying on us.

Y: Well Don, we can easily find out if you're right.

D: We can?

Y: You see, Don, two-way mirrors work on a principle of light intensity. If the light      intensity is the same on both sides of the glass, the mirror will look like a normal      piece of glass. But when the light is bright on one side, and much darker on the other,      the glass will look like a mirror to the people on the brighter side. The same thing      happens when you try to look out a window at night. Even though people on the      outside can see you, you can only see your reflection.

D: So a two-way mirror is just a simple sheet of glass?

Y: No. Like all mirrors, two-way mirrors have a reflective coating. However, while the      reflecting coating on regular mirrors is dense and returns all the light that strikes its      surface, the reflective coating on two-way mirrors is more sparse. This means it      reflects about half the light that hits its surface and allows the other half to pass      through to the other side, just like a window.

D: Hmm! So if we turned off the lights, our side of the mirror would be darker than the      other side, and we'd be able to see if anyone's sitting there.

Y: Right. Let's try it! See, there's nothing to worry about. It's only a mirror.

D: Unless they heard us talking, and turned off their lights.

Y: Feeling a little paranoid, Don?  

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