Silica aerogel looks extremely odd. Put it against a light background and it disappears almost entirely. In this sense, it is harder to see—more invisible, even—than normal glass despite being less transparent. When light passes through glass, its path is distorted slightly—it is refracted—and the degree of distortion is known as glass’s refractive index. In the case of aerogel, because there is simply less of the stuff, light’s path is hardly distorted at all. For this same reason, there is no hint of reflection on its surfaces, and because of its ultra-low density it appears to have no
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