Selective Scattering
better at scattering certain wavelengths than others

Scattering is a rapid process whereby light is actually absorbed by a particle and then quickly emitted in another direction.

Scattering particles can be air molecules, dust particles, water droplets or pollutants, which scatter incoming sunlight (or moonlight) in all directions, similar to how a pinball bounces around in a pinball machine. During the scattering process, no energy is lost or gained, so no temperature changes occur.

Photograph by Ron L. Holle

Selective scattering occurs when certain particles are more effective at scattering a particular wavelength of light (Rayleigh scattering). Air molecules (oxygen and nitrogen, for example) are small in size, and thus more effective at scattering shorter light wavelengths (blue and violet), producing blue skies visible on a clear sunny day.

Photograph by Ron L.Holle

When the sun is low on the horizon and the atmosphere is loaded with particles, only longer wavelengths of light (yellow and red) are able to penetrate the dense atmosphere. Other wavelengths of visible light are simply scattered away and never reach our eyes, producing in a colorful sunset at the end of the day.

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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.