Imagine if instead of installing curtains or blinds, you could simply adjust the opacity of the glass in your windows. Not only would this allow you to vary the amount of privacy they provided, but it would also let you determine how much sunlight got through, keeping rooms from overheating during the day yet still letting light in later on. Well, that's how the various types of smart windows work. Researchers at Harvard University have now developed one of their own, which they say is simpler and cheaper than what's come before.

Generally speaking, smart windows rely on electrochemical reactions in order to change transparency. This can make for an expensive manufacturing process, along with the use of sometimes-toxic substances.

The Harvard technology is different. It incorporates a sheet of regular glass or plastic, sandwiched between two clear elastomer layers that have been sprayed with silver nanowires. Those wires are small enough that they don't affect light transmission on their own.

However, when an electrical current is applied (simply by flicking a switch), the nanowires on either side of the glass are drawn towards one another. This causes them to compress the elastomer layers, distorting them in the process. As a result, the window goes from being completely clear to taking on a cloudy frosted-glass appearance.

The whole process takes less than a second. Additionally, it's possible to tweak how opaque the window gets, simply by varying the amount of voltage applied.

"Because this is a physical phenomenon rather than based on a chemical reaction, it is a simpler and potentially cheaper way to achieve commercial tunable windows," says Prof. David Clarke, who led the research along with postdoctoral fellow Samuel Shian.