A post submitted by CGI member ScienceTruth.

***************************************

The Winnemucca Institute for Advanced Studies presents

Why is the Sky Blue?

It is a complex of interactions between the arriving light energy from our Sun, our Earth's atmospheric composition, and how our eye perceives and translates the energy of light.

If you've been fortunate enough to be in a place on a clear day when the sky is empty of clouds, haze, and atmospheric pollution, and the Sun is at an oblique angle or larger, to overhead, you will have seen a most incredibly beautiful blue sky. A sky that will bring joy into your heart and tears to your eyes at the wonderment of how and why this world (and this reality) can be so amazingly beautiful and pure, and does anyone besides us appreciate and wonder at this beauty!

The energy radiations from our Sun arrive from all across the energy spectrum, but our eyes are only sensitive to a small slice of that total spectrum, that of visible light. Our planet Earth's atmosphere is mostly nitrogen and oxygen, two atoms that are both smaller than one wavelength of visible light; but they will however interact with that light energy, causing it to 're-radiate' at differing angles depending upon its energy level. Shorter wavelength/higher energy light, violet and blue, will re-radiate at a larger angle; while longer wavelength/lower energy light, red and orange, will re-radiate from atmospheric constituents at a lesser angle from its direction of arriving incidence. In this way, blue light is 'scattered' more than red light is scattered. Thusly, when you look up, and the Sun is several hours away from noon, the red light end of the spectrum travels onward, and you don't see it, as it is not being re-radiated toward you from overhead, but you do see the blue light that is being re-radiated towards you from overhead.

You might ask why the sky doesn't look purple or indigo, since purple and indigo are scattered even more than blue. This is because our eyes are much less sensitive to purple and indigo than they are to blue, and so the 'cones' in our eyes don't translate nearly as much nerve-impulse electrical energy from those wavelengths as they do from the blue wavelengths, and then send that translation into the optic nerve to our brain for interpretation.

If we were in outer space the sky would look black, because there is no 'atmosphere' to scatter any light back to us, all the stars and our Sun would look 'white', the color of all the visible light spectrum of radiations summed together. Some stars are predominantly red or blue in their visible light energy output, and so those stars would look more red or blue than white, like Betelgeuse or Rigel. If we were on a planet orbiting Rigel, a blue-white star, and our body was adapted to a heavy methane atmosphere, like on Saturn's moon Titan, we'd be looking up and seeing a 'Blue Sun in a Yellow Sky' !

Information provided herein by the Winnemucca Institute for Advanced Studies is for educational purposes. Our ‘Man on the Street Series’ of informative Science Papers is designed to provide a semi-technical answer to everyday experiences.