astronomy

Why is the sky blue?

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  1. The sky is blue because shorter-wavelngth blue radiation is scattered more by the air than other colors.

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  2. The sky is blue because light from the sun, which contains all colors, is scattered by the air. It turns out that blue light is scattered more than other colors. If there were no scattering, the sky would be black.

    The molecules in the air scatter all colors of light, but tend to scatter shorter wavelengths more than longer wavelengths. Light is a wave, and the color of the light corresponds to different wavelengths. A rainbow separates light according to wavelength: red, orange, yellow, green, blue, violet. Air molecules are very small, so they scatter light this way, as do very small dust particles and water droplets. This is called Rayleigh scattering, named
    after the physicist John Strutt (Lord Rayleigh) who first explained why the sky is blue.

    Rayleigh scattering is the scattering of a wave from a particle whose diameter is much smaller than the wavelength of the wave. If the wave is a light wave, and the particle is a very small dust particle or water droplet, this condition is satisfied. (It is also satisfied by random density fluctuations in the air, so that the local air acts like the surrounding air with the extra air acting like a particle.)

    You can imagine the electric field in the light wave causing charges in the particle to oscillate. Those oscillating charges radiate light at the same wavelength, but in all directions. This radiated light adds to the light originally passing through. Rayleigh showed that the net effect is the re-radiation of light at "sideways" angles.

    Moreover, he showed that shorter wavelengths (shorter, but still large compared to the molecules) are scattered more strongly. The scattering strength is proportional 1/{fourth power of the wavelength}. This strong wavelength dependence arises from the nature of electric dipole radiation. The wavelength dependence means, for example, that blue light scatters about ten times as strongly as red light.

    Large particles scatter all colors equally well; that is why clouds are white. They consist of large water drops or ice crystals. Likewise, when the sky is hazy, it is white because the particles in the air (aerosols, not molecules) are large and scatter light as clouds do.

    This was originally written by a recent associate whose name I forget.

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