Why is the sky blue?

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Contents 1 Question 2 Answer(s) 2.1 Answer 1 2.2 Asnwer 2

[edit] Question

Why is the sky blue?

[edit] Answer(s)

[edit] Answer 1

The sky is blue due to Raleigh scattering

To give a very simple answer, its because air is blue. To give a more extended answer, an explanation of Rayleigh scattering is necessary.

The sun exist (almost) as a blackbody, which means that it radiates energies at all wavelengths, from very long wavelength radio waves, to extremely short gamma rays. Because the sun isn't a perfect blackbody, it shines more brightly at certain wavelengths than others. For instance solar gamma radiation is not as strongly radiated as in visible wavelengths. Our atmosphere is pretty good at absorbing certain wavelengths as well. Most of the ultraviolet radiation is absorbed by the ozone layer. The visible wavelengths of light, which our eyes have adapted to see, pass through largely unimpeded. However, blue light is scattered by dust and gas particles in the air. Rayleigh scattering occurs when light reflects off of particles smaller than the wavelength of incident light. In fact, violet and long wavelength ultraviolet light is more strongly scattered, however our eyes are not as sensitive to these wavelengths as we are to blue light.

Sunsets (and occasionally sunrises) scatter light at longer wavelengths because of the increased thickness of atmosphere the light travels through. As the earth rotates, the angle at which the light enters the atmosphere with respect to the ground decreases. At high noon, the light passes through the thinnest portion of atmosphere it can, for that particular day. About an hour before (astronomical) sunset, the light becomes scattered at longer and longer wavelengths. Looking low to the horizon, the sky appears not blue, but white, again because of the thickness of the atmosphere at low angles. The light around the western horizon during sunset turns yellow, orange and red, corresponding to longer wavelengths. At some point, the sun falls below the horizon, and darkness sets in.

As an explanation of the simple answer, consider the appearance of liquified air. Using cryogenic techniques, it is possible to take gaseous air and cool and thus condense it into a liquid. Seen through a clear flask, liquified air (consisting of about 78% nitrogen, 21% oxygen and the remainder mostly being argon, neon and carbon dioxide), appears blue. To explain why the room around you doesn't have a bluish hue, consider a muddy pond. Looking through the thickest portion of the pond, the water appears cloudy and brown. However, if one was to take a small sample, say a drinking glass worth, the water appears clear.

[edit] Asnwer 2

The atmosphere is mostly composed of nitrogen and oxygen. When light from the sun is difused by nitrogen and oxygen, all but the shortest wave lengths pass through the molecuels. These short waves are bounced around in all directions, causing the light to scatter all around you. Since blue has the shortest wavelenght of the visable light spectrum, you see more of it than any other color.