Answered By: Doug Bolden
Last Updated: Oct 07, 2015     Views: 228

One time a student, spotting our "Ask Us Anything" logo, asked the librarian on duty at the time: "Why is the sky blue?" Whether the student was serious or not, he got to hear about Diffuse Sky Radiation and related topics. For the absolutely tl;dr crowd: different wavelengths of light are scattered differently by the atmosphere and blue light, scattered the most, dominates the spectrum. For those who like a little more answer, you can see the answer through NASA's SpacePlace:

All light travels in a straight line unless something gets in the way to—

  • reflect it (like a mirror)
  • bend it (like a prism)
  • or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

To make it a litle more complicated, part of the equation is in the human eye itself, as explained by Philip Gibbs (as posted on The Original Usenet Physics FAQ)

When we look up at the sky, the red cones respond to the small amount of scattered red light, but also less strongly to orange and yellow wavelengths.  The green cones respond to yellow and the more strongly scattered green and green-blue wavelengths.  The blue cones are stimulated by colours near blue wavelengths, which are very strongly scattered.  If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge.  However, the most strongly scattered indigo and violet wavelengths stimulate the red cones slightly as well as the blue, which is why these colours appear blue with an added red tinge.  The net effect is that the red and green cones are stimulated about equally by the light from the sky, while the blue is stimulated more strongly.  This combination accounts for the pale sky blue colour.  It may not be a coincidence that our vision is adjusted to see the sky as a pure hue.  We have evolved to fit in with our environment; and the ability to separate natural colours most clearly is probably a survival advantage.

In other words, the sky is blue partially because our eyes are more attuned to seeing it as blue.

The reason this is an interesting question is because it seems like such a simple one, but really we are only something like a century and a half into a world where someone has answered it. John Tyndall and  John Strutt, Lord Rayleigh were a big part of answering it (Strutt answered it in greater detail, Tyndall had the first experiment to study it, so the is commonly named after Strutt, or more specifically after his title of Lord Rayleigh). You can read Strutt's 1899 article, "On the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky", originally published in Philosophical Magazine, series 5, volume 47. 

In fact, you can read Rayleigh's first article on the topic, "On the light from the sky, its polarization and colour", in our stacks! We have the Scientific Papers of Lord Rayleigh, in the three volume edition, at call number QC3 .R26, which is on the third floor, north wing. Said article (and its more or less immediate follow up, ""On the scattering of light by small particles", are on pages 87 and 105 of the first volume of the papers.

Interestingly, the first article includes some discussion about another theory from that time, that the upper atmosphere was made up of water bubbles. To see some write-up about other historical theories about the blue sky phenomenon, you can read, among others, Pesic, Peter. "The sky is falling: Newton's droplets, Clausius's bubbles and Tyndall's 'sky matter'". European Journal of Physics, 26.1 (2005).

It is more complicated than just Rayleigh Scattering, the color of the sky and the way light comes through (e.g., see also Mie Scattering), but you have the basic idea, now. 

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