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Big windows provide light, and a view, but they don't always do much for energy efficiency or privacy. The glass transmits heat to the outside in winter and traps it inside during the summer. The only real solution: curtains or blinds.

But now there's a glass that changes, chameleon-like, from opaque to transparent, and can be adjusted for different wavelengths of light. It could boost energy efficiency in buildings with large glass facades, freeing homeowners from the chore of picking window treatments.

The glass is the brainchild of scientists at Lawrence Berkeley National Laboratory (LBNL) in California and the Autonomous University of Barcelona. It's made of niobate, a compound composed of the element niobium and oxygen. The ingredients get mixed together with nanometer-sized crystals of indium tin oxide, or ITO, which is used in touch screens to register when a finger contacts the display. The scientists' experiments appear online in the Aug. 14 issue of the journal Nature.

The resulting material is called a glass because it has the characteristic structure of a glass: the molecules are all jumbled around, without a clear pattern, like in a liquid. But unlike a liquid, glass doesn't flow. Ordinary window glass is made of silicon dioxide mixed with other chemicals, such as sodium oxide (Na2O), magnesia (MgO), lime (CaO) and alumina (Al2O3), which give it extra strength.

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