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Name: Kevin
Status: student
Grade: 9-12
Location: PA
Country: USA
Date: Fall 2011

As we all know, the sky can appear to change from purple, blue,orange, and red, but why is it that green and yellow are left out? With the exception to any weather phenomena.

Hi Kevin,

The Physics involving the scattering of light in our atmosphere is called Rayleigh Scattering.

This scattering is caused by the particles or gases in our atmosphere and is defined to be inversely proportional to the 4th power of the wavelength. Since it is inversely proportional, the shorter wavelengths (blues and greens) would experience the most scattering.

Then why do not we see green?

Probably because the blue spectrum is much wider than the green spectrum, and so blue is heavily weighted and dominates in the scattering.

The other side of the spectrum of light, the reds and yellows, are visible when the sun is near the horizon, like during sunsets. This is because, near the horizon, most of the light we see has been filtered away, or scattered away, and all that is left are the longer wavelengths - the yellow sun and the red and orange sky. So, you actually do see yellow from our horizon, because in outer space, the sun is actually white.

In short, it is the scattering of the dominate, shorter wavelengths that give us blue skies; and at sunsets, if all the blue is scattered, what is left are the longer wavelengths, the yellows, reds, and oranges. Only the sun appears yellow, but it is really the sky that is yellow and red and hence orange too!

My Physics Professor once said, and I will never forget it, "if you are experiencing a beautiful red-orange sunset...then someone, somewhere, on another part of the world, is experiencing blue skies."

Hope that helps! -Alex Viray

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