SKY IN A BOTTLE

Why is the sky blue? Here's the story of how the answer was discovered.

Pesic (Tutor and Musician-in-Residence/St. John's Coll., Santa Fe) begins with early civilizations. Surprisingly, the ancient Greeks and Chinese ignored the question. Aristotle recognized that air itself is colorless, but suggested that in great quantities it might have a color of its own. An alternate theory arose in the 10th century A.D., when the Arabic savant Ibn al-Haytham investigated the reflection and refraction of light by particles in the air, in a treatise later borrowed and reworked by Roger Bacon. Seven centuries later, Descartes attempted to show mathematically that vortices in the air reflect the rays, both to produce the white of clouds and the blue of clear sky. Newton, rejecting the vortices, showed that sunlight includes all colors of the spectrum, but (convinced of the particle nature of light) missed the mechanism by which one color is scattered in preference to others. In 1760, Pierre Bouguer published the first approximation to the true answer, the separation of various colors by air molecules. The theory failed to gain acceptance immediately, reality of molecules still being unproven. This laid the groundwork for the British physicist John Tyndall to study the effect of extremely small particles on light, though he ultimately rejected the idea that molecules themselves could be responsible. His work inspired William Strutt, Baron Rayleigh, who established the relationship between the wavelength of light and its scattering by small particles. Those results—and the existence of molecules—were confirmed in Einstein's doctoral thesis. Pesic presents the various theories in clear form, suggests simple experiments that illustrate the scientific points and reproduces several letters between some of the principal scientists.

A fine example of just how much scientific treasure even the simplest of questions can unearth.