A Moment of Science

How Two-way Mirrors Work

When you watch cop shows on TV you often see suspects being interviewed in a room with a two-way mirror. In this Moment of Science, we’ll learn how they work.

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.

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.

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