At some point in life, almost every child on Earth asks, “Why is the sky blue?” Today we know the answer—the sky appears blue because molecules in the air scatter blue light from the sun more than they scatter the other wavelengths, or colors. But it took a long time for scientists to figure that out.

It wasn’t until 1859 when the phenomenon was first correctly explained by Irish physicist John Tyndall, although it was Lord Rayleigh, who studied it in more detail a few years later, after which the phenomenon of Rayleigh scattering is named.

A modern cyanometer in Ljubljana, Slovenia. Photo credit: artecitya.eu