This week's experiment is based on what many people call a "two way mirror." This is a mirror which seems to be a perfectly normal mirror when viewed from one side, but if you are behind the mirror, you can see through it as if it was a pane of tinted glass. These mirrors are often used in department stores, to allow security staff to keep an eye on you without you knowing.

How do these mirrors work? With a normal mirror, you have a pane of glass with a shiny, silver coating on the back side. Light passes through the glass, reflects off the silver coating and bounces back through the glass to your eyes. The glass protects the delicate silver coating.

For a two way mirror, the front surface of the glass reflects some of the light and lets some of it pass through. In order for it to work properly, the light on the "mirror" side must be bright and the light on the back side must be dim. Enough light is reflected for the mirror side to appear to be a normal mirror, but enough light passes through for the person on the other side to easily see through it.

To get an idea of how this works, you will need"

- a clean window
- a dark night

Wait until it is very dark outside. Find a window that does not have any bright lights outside. Turn on the lights in the room, so that it is very bright. Now, try to look out the window. Instead of seeing outside, mostly you will see your reflection. Now, go outside and look into the same window. This time, you see through easily. Keep in mind that this is just a normal window pane. By adding the reflective coating, it works much better.

There is an easy test to see if that mirror in the store is one way or two way. Touch your finger to the mirror. If it is a normal mirror, then the glass will separate your fingernail from its reflection. There will be a tiny gap between the two. If it is a two way mirror, then the image of your fingernail is reflected from the front surface and will touch your fingernail without a gap. You can try this by using a regular mirror and then using your reflection in the window pane on a dark night. I have not had a chance to test this extensively yet, but so far it seems to work very well.

I should mention that some expensive mirrors that are used in optical equipment are silvered on the front surface, so that the light is not distorted as it passes through the glass, but I don't think there is any danger of finding one of these in a department store.