Aerogel is made of the same material as glass, silica. Consequently it appears clear:
Held to a darker background though and it's true color is revealed... blue:
The reason is a process called Rayleigh Scattering, and it's the same reason the sky is blue. The sun looks yellow to us because that's its color and we're looking at it directly (and this is dangerous so don't do it very long!). However the further from the sun you look, the more that sunlight has to reflect off of air molecules to get to our eyes. It turns out this process is far more effective at shorter wavelengths than longer ones. Recall Roy G. Biv, which runs from longer to shorter:
Red - 650nm (longer)
Orange - 590nm
Yellow - 570nm
Green - 510nm
Blue - 475nm
Indigo - 445nm
Violet - 400nm (shorter)
Sunlight includes a bunch of other colors but less strongly than yellow, so overall it looks yellow to us. However, when sunlight is scattered all the shorter wavelength light, like blue, will overpower the longer wavelengths, like yellow, and the net effect is a blue sky... and also blue aerogel.
We can test this using different color lights and comparing the color of the aerogel under various degrees of scatter. First we'll use aerogel like a lens, almost directly between our eyes and the light. Then we'll set it aside and look at it, which forces the light from the lamp to scatter within the aerogel and eventually come back to our eyes. Below we have crops from photos I made of both scenarios. I then used "Average Color Seeker" to get the average color of each image and made a chart of the difference, as a percentage, in Excel.
| | Direct | Indirect | Chart |
|---|
| Red |  |  |  |
|---|
| Yellow |  |  |  |
|---|
| Green |  |  |  |
|---|
In all the images the longer wavelength, red, goes down while the shortest one, blue, goes up. This points to the materials in the aerogel being smaller than the wavelength of visible light, which is the prerequisite for Rayleigh Scattering. In the dye experiment we were able to establish the majority of the pores to be between 969 and 1711 picometers, or between 1 and 2 nanometers, roughly 398 nm shorter than the shortest visible wavelength of light, violet.
