The Science Behind Two-Way Mirrors

In a word, two-way mirrors are strategic. Their appeal lies in the fact that they look like a regular mirror from one side, allowing an individual to surveil another. This, however, is not a magic trick. In fact, two-way mirrors undergo a similar creation process as regular mirrors, with only slight modifications to the production. To create a run-of-the-mill mirror, a metallic, reflective coating is placed behind a piece of glass. Nowadays, aluminum is commonly used in this process. When light hits the metallic coating, it bounces back into the eyes of the viewer, allowing them to see their own image.

In contrast, a two-way mirror uses a much thinner reflective layer. Per HowStuffWorks, this a referred to as a "half-silvered surface." This means that the glass has fewer reflective molecules than an average mirror. Simply put, only half the glass is covered in reflective molecules. The products used in this process are usually semi-transparent aluminum on a piece of acrylic.

This thinner layer only reflects some of the light — usually around 50% of it — while the rest of the light is absorbed into the mirror. Both sides of the mirror are actually created in the same way, meaning that, from either side, about 50% of any light is absorbed while 50% is reflected. Ultimately, this creates one transparent and one reflective side, and a two-way mirror is born. So why can viewers only see through the mirror from one side of the pane? It's all about understanding the light.

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