Why is the sky blue?

Yesterday I was watching Barney the dinosaur sing songs with his friends on tv (I have 2 kids) and part of the lyrics of a song were why is the sky blue? So as became numb with hours and hours of Barney songs stuck in my head, I started to think about the science behind why the sky is blue. I will admit that I didnt know the answer to that question but I was determined to find out.

Why is the sky blue?

Understanding why the sky is blue is about having an grasp on why things can appear a certain color. Visible light is made up of all colors: red, orange, yellow, green, blue, violet, etc. An object appears blue if it scatters more blue light than the other colors of light. When I say scatters I mean it reflects in all directions. For solid objects, those other colors are absorbed rather than scattered, and so whatever color is most scattered by the object is the color the object appears. For the atmosphere though, those other colors pass right through without being absorbed to the ground. Some of the blue light is scattered off the molecules of air making the atmosphere appear blue.

Why blue? The size of those air molecules are close to the wavelength of blue light (about .0004 millimeters in diameter.)

When flying in an airplane at 34,000 feet, you are above 75% of the atmosphere. If you look out the airplane window and look up, you will notice that the sky is a much darker blue than you are used to seeing. This is because there is less air to scatter the blue portion of the incoming sunlight. An astronaut in space is above the Earths atmosphere so outer space appears black.

If you were in outer space

If you were in space or on the moon, the sun would look white. In space there is no atmosphere to scatter the suns light. On Earth, some of the shorter wavelength light (the blues and violets) are removed from the direct rays of the sun by scattering. The remaining colors together appear yellow.

Why is the sunset red?

As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.

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