Why is the Sky Blue? The Surprising Answer!

The classic example of childhood curiosity is the question "Why is the sky blue?" In fact it is said that Thomas Edison pestered his parents with this question. So what is the answer to this question? The short and technical answer is summed up in two words: Rayleigh scattering (impress your friends with that one!). Rayleigh scattering is a process by which light of shorter wavelengths is scattered more than that of longer wavelengths by the molecules of gas in our atmosphere.

Scattering causes light to be deflected from the direct path, with some of it bouncing off particles in the atmosphere such as molecules and small dust particles and spreading out so that it eventually reaches our eyes from many different directions, not in line with the original beam of light. So, when we are looking at the part of the sky not in direct line with sun, we are seeing only the light that is scattered. If the amount of scattered light is wavelength (or color) dependent, then it will be the color of the predominantly-scattered light.

Wavelength is a property of light that is perceived by our visual system as different colors - with the shorter wavelengths being towards the blue-violet end of the spectrum, and longer wavelengths being toward the orange-red end of the spectrum. The full spectrum is the colors of the rainbow, in order from longer to shorter wavelengths as: red, orange, yellow, green, blue, indigo, violet.

So, we can see that blue is one of the shorter wavelengths of the visible spectrum of light. But wait - it is not the shortest! Didn't we just say that violet is the shortest wavelength of the visible light spectrum? So why doesn't the sky appear to be violet? In a very real sense the true color of the sky is violet, if we measure the color of the sky by the wavelength of maximum energy coming from it.

The answer lies in the response of the human visual system to different colors - it is not uniform. In fact, the eye is most sensitive to the middle portion of the spectrum, the green-yellow colors. It is less sensitive to the shortest wavelengths. So in particular, it is more sensitive to blue than it is to violet (or indigo for that matter). That is why, even though violet is scattered more than blue, the sky appears blue to us! It is the balance between the response of the human visual system and the amount of scattering in the atmosphere.

So the next time someone asks you why is the sky blue-surprise them with the answer: the sky is not blue. And then impress them by explaining Rayleigh scattering.