This Raleigh scattering, as it is called, is very slight indeed, so you need an enormous volume of gas molecules to see it: the sky works but a room full of air doesn’t. Put another way, any one bit of the sky doesn’t look blue but the whole atmosphere does. But if a small amount of air happens to be encapsulated in a transparent material that happens to contain billions and billions of tiny internal surfaces, then there will be sufficient Raleigh scattering off these surfaces to change the color of any light that passes through it. Silica aerogel has exactly this structure, and this is where
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