A group of MIT scientists have created a new material that can be both a mirror and a window, and no it's not a one-way mirror.
This new material can filter light depending on the direction of the light beams. In the image above light that hits from one angle goes straight through (white beam) but light that hits the material at different angle is reflected back (red beam). For designers it might make for interesting new tricks for walls or new forms of windows.
To filter light one must alter either it's frequency or polarization. In terms of frequency, stained glass windows are a good example, where the glass lets specific wavelengths pass through.
Polarized glasses, like the 3D glasses you wear at the movies, are able to let light through that oscillates in a specific way. But the idea of filtering light based on the direction it comes from has always been tough.
The scientists built a material with 84 very thin layers that alternate between glass and tantalum oxide (used in most car electronics, cell phones, etc.). Light passes through the layers but some gets reflected. However at a certain angle, called the Brewster angle, polarized light can pass directly through both materials and so it looks like there is nothing there at all.
This new material is significant because it can be reflective just a like a mirror, but then with a turn it can let light through just like a window. See video below:
One challenging caveat is that the mirror only works in liquid that has the same refractive index as glass. Air has a different refractive index to glass and tantalum oxide and this wrecks the trick.
However, the researchers say you can swap the glass with something that has the same refractive index as air—like an aerogel—and this could work. I think using the aerogel, a.k.a. frozen smoke, makes the entire project that much more exotic. And just super cool.