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Why does Earth’s atmosphere scatter sunlight in many directions?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Sunlight is scattered in Earth’s atmosphere because the electric and magnetic fields of incoming electromagnetic radiation exert forces on the bound electrons of atmospheric atoms and molecules. These electrons oscillate at the frequency of the incoming wave and, acting as sources of radiation, reradiate the energy in various directions.1

Unlike in dense materials like glass, where reradiated waves undergo destructive interference that limits scattering, the molecules in the upper atmosphere are distributed such that interference does not prevent light from being redirected. This process, known as Rayleigh scattering, is highly dependent on frequency; shorter wavelengths (such as blue light) are scattered much more strongly than longer wavelengths. Consequently, when we look at the sky away from the Sun, we see this scattered light, which is why the sky appears blue. Conversely, during sunrise or sunset, the light travels through a longer path of air, causing the shorter wavelengths to be scattered out of the beam and leaving the remaining light to appear reddish.1234