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A one-way window is in theory a piece of clear material, most commonly glass, through which one can only see in a single direction. Common uses for a one-way window would be businesses or residences which hope to maintain some level of privacy by blocking outside people from seeing inside the office or home. The reason this is a theory is that unless some kind of film is applied to smooth glass, a one-way window still cannot block the view if there is light on the inside and it is dark on the outside.
Without the use of energy, it is impossible to create a one-way window. This is because it violates the second law of thermodynamics, which states that chemical differences will tend to equilibrate. Therefore, energy will still transfer from one side of the glass to the other. While this is the case, objects lit from the inside can be seen from the outside. As long as an equal amount of light is in both places, a one-way window is possible.
An exception to the false notion of a true one-way window is electric privacy glass. This glass is made with a layer of liquid crystal film and uses energy to change the direction of light. While not cost-effective for use as a one-way window in most cases, the technology has been used for workshops and conference rooms, among other places. Electric privacy glass can transform from transparent to opaque in a matter of milliseconds. It is important to use incandescent light rather than fluorescent light when using electric privacy glass.
If the desire for a one-way window still exists, one option is to coat the glass for privacy. The most common and inexpensive way is to use a thin film which can be placed over the glass, allowing someone on the inside to see outside but not vice-versa. This film is now widely available in stores such as home improvement warehouses or online. Numerous patterns are available like camouflage or stained glass.
If privacy is the foremost concern for a window treatment, one might consider frosted glass. This type of glass still allows light to be transferred through it, but it obscures the vision from both sides. Mirrored glass is often used for two-way mirrors, especially in police interrogation rooms, and one side can see through while the other side sees only a reflection of the room. Even still, there should be some kind of light differential between the two since it will work more effectively if it is brighter on the side that is mirrored.