Why is the sky blue and not blue & purple? (Beginner)

If high frequency light is scattered, why do we see a blue sky instead of a blue and purple or violet sky?

The sky appears blue for a combination of two reasons. Before white light reaches the Earth's surface, the light waves collide with and bounce off of the nitrogen and oxygen atoms in the atmosphere. Different frequencies of light (in other words different colors) are scattered differently. Higher frequencies (blue and purple) are more easily scattered and thus bounce around in all different directions more than lower frequencies (red or orange) do. The scattering of high frequencies alone would cause the sky to appear blue and purple, but our eyes work better at frequencies near the middle of the spectrum (yellow and green). Since the color blue is closer to yellow or green than purple is, the sky we see appears blue.

This page was last updated June 27, 2015.

About the Author

Sabrina Stierwalt

Sabrina Stierwalt

Sabrina was a graduate student at Cornell until 2009 when she moved to Los Angeles to become a researcher at Caltech. She now studies galaxy mergers at the University of Virginia and the National Radio Astronomy Observatory in Charlottesville. You can also find her answering science questions in her weekly podcast as Everyday Einstein.

 Twitter: @galaxygirlguru
 Website: www.astro.virginia.edu/~srs5vn

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