Why is the sky blue?

A question that I get very often is why did I ever return to Greece to do science. It is indeed hard to explain why would a young scientist leave an educational and technological paradise such as Yale University to seek a better future in a country on the verge of default. So, I usually start by explaining the ties to my family, my friends, the ease to speak my own language, etc. But then again only one picture could actually summarize it 🙂

Elia

Elia beach, Mykonos

In a country with such blue skies, I often get bombarded with questions from my non-scientist friends such as: “Why is the sky blue? Why is the sunset red?”. So here it is in very simple words:

Light

Light looks white, but it is actually a combination of colors (you’ve all see a rainbow right?). These colors are in order: red, orange, yellow, green, blue, indigo and violet. These colors all have different energies, violet has the highest energy and red the lowest. Light travels in a straight line when it is undisturbed, such as in space. What happens when light enters the atmosphere?

Atmoshpere – Blue Sky

The atmosphere is composed by particles such as dust and water that can be seen with the naked eye, but also smaller particles such as oxygen and nitrogen that are so small that we cannot see them. When light enters the atmosphere it hits these small molecules such as oxygen (21%) and nitrogen (78%) (gases). Gases then tend to absorb not all of the light but only part of it. By “part of the light” I mean that they absorb only one out of 7 colors. It so happens that this color is blue. After a while, the gas molecules give off this blue light in all different directions in the atmosphere. The blue light is scattered everywhere you look in the atmosphere. This is why the sky is blue. This is also why the sun looks yellow (remember light is white). Because if you take off blue from the rainbow, all the remaining colors together look yellow.

Atmosphere – Red Sunset

So why isn’t the sky blue also at sunset? At sunset, the sun is close to the horizon and light needs to pass through a much longer path through the atmosphere to reach you than at noon. Close to the horizon different particles (such as aerosols) are concentrated. It so happens that these aerosols absorb the red light instead of the blue for a while and then give off this red light at all possible directions such as oxygen and nitrogen did for the blue light. Therefore we can enjoy red sunsets and blue skies…

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Posted on July 18, 2011, in science facts. Bookmark the permalink. 3 Comments.

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  1. Pingback: A Day in the Life of a Computational Chemist | Life is Chemistry

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