Businesses or homes use one-sided windows for appearance and privacy.
A one-sided window is, in theory, a piece of transparent material, most commonly glass, through which you can only see in one direction. Common uses for this type of window are businesses or homes that want to maintain some level of privacy, preventing outsiders from seeing into the office or home. The reason this is a theory is that unless some sort of film is applied to smooth glass, a one-way window still cannot block vision if there is light inside and if it is dark outside.
Mirrored glass is often used for two-way mirrors, especially in police interrogation rooms.
Without the use of energy, it is impossible to create a one-sided window. This is because it violates the second law of thermodynamics, which states that chemical differences tend to balance out. Therefore, energy will still be transferred from one side of the glass to the other. As long as this is the case, objects lit from within can be seen from the outside. As long as there is an equal amount of light in both places, a one-way window is possible.
One exception to the false notion of a truly one-sided 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 to use as a one-sided window in most cases, the technology has been used for workshops and conference rooms, among other places. Private electric glass can change from clear to opaque in a matter of milliseconds. It is important to use incandescent light instead of fluorescent light when using electric glass for privacy.
If the desire for a one-sided window still exists, one option is to coat the glass for privacy. The most common and cheapest way is to use a thin film that can be placed over the glass, allowing someone inside to see the outside, but not vice versa. This film is now widely available in stores such as remodeling sheds or online. Various patterns are available, such as camouflage or stained glass.
If privacy is the main concern for a window treatment, one might consider frosted glass. This type of glass still allows light to be transferred through it, but obscures 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 only sees a reflection of the room. Even so, there must be some sort of light differential between the two, as it will work more effectively if it is brighter on the side that is mirrored.