You've probably seen pictures of Greek villages, where every house is painted bright white. The paint reflects the intense sunlight of the Mediterranean. And it works pretty well to keep the houses from heating up in the sun. But it doesn't actively cool them.

To understand why, consider what the white paint does. Here's how optical scientist Xiaobo Yin of the University of Colorado describes it: "It's a mirror for the sunlight, it's also a mirror for the radiation as well." And it might seem like reflecting all those incoming rays would be a good thing. But the benefit is limited. Because what it really means, Yin says, is that the houses, well, "they don't release much energy." 

Release more energy, again in the form of infrared radiation, aka heat—and you effectively get free A/C.

So Yin and his colleagues built a material that does exactly that: reflects visible light, but also emits infrared wavelengths. Which gives it the power to actually cool. It's two layers: a top layer of polymer, packed with glass beads just eight microns across—so they can absorb and then emit infrared radiation. And a silver coating on the bottom.

The material reflects 96 percent of solar radiation—a slight improvement on white and silver paint. But the game-changing part: it also has a cooling power of about 100 watts per square meter. Translation: enough to cool a frying-pan-sized amount of water to about 15 degrees Fahrenheit cooler than ambient temps. Even while sitting in the sun. The finding is in the journal Science.

That cool water example has wide applications. "This water can be used to cool a house, cool a data center, or even cool a thermoelectric power plant." That's Yin's colleague at the University of Colorado, Ronggui Yang. And he says it's a cheap way to cool things down. Just don't put the material directly on your roof. "It also cools down during the wintertime. And you do not want that to happen." No. Definitely... not cool.