Why is the sky blue? It's a question that most kids ask their parents at some point, but does the average adult really even know the answer? In order to truly understand why the sky is blue we must first review a couple of basic concepts about light and the atmosphere.
The light from the sun is white light. White light contains all of the colors of the rainbow. You may remember ”ROY G BIV” from science class. The mnemonic device, Roy G Biv, helps us remember the colors of the rainbow or visible light spectrum which are Red, Orange, Yellow, Green, Blue, Indigo, and Violet. These colors travel through the air like waves and they have different wavelengths. Red has longest and lowest frequency wavelength (let’s think of it as being long and strong). Blue is one of the smallest and highest frequency wavelengths (he is the little guy, the pushover).
As the white light from the sun shines down, it travels through the atmosphere but not without encountering obstacles along the way. The white light bumps into particles of solid and gas in the atmosphere. When the different wavelengths of light hit these different particles, some of the colored light is absorbed and then re-dispersed and scattered all about. This happens most often to short wavelengths of light like blue, remember, he is the pushover and gets pushed around by those particles a lot. Longer (and ‘stronger’) wavelengths of light with lower frequencies like Red, why he is so strong he moves right through the atmosphere and all of the particle matter practically unscathed. This phenomenon was first observed by John Tyndall in 1859 and became known as the Tyndall effect. It was then studied further by Lord Rayleigh and is most commonly known as Rayleigh Scattering to physicists today. However, Rayleigh Scattering wasn’t fully accepted until Einstein confirmed it by applying a detailed formulation to calculate the scattering of light by molecules in 1911.
So, since we have all of this blue light being absorbed and bounced around all over the atmosphere and in every direction, it becomes the main color dispersed in the atmosphere. And that is the short answer (was it really?) to why the sky is blue!
Go get a latte, come back, and I will talk to you about sunsets next.
You there? Ok, great. Let’s get started. First, before we bid the topic of the bright blue sky adieu, some of you that have already had your coffee before you started reading this article may have caught a little flaw in the explanation of the sky being blue. I said that the smaller, high frequency wavelengths of light are the colors which get thrown around the most by particulate matter in the atmosphere, that they are the pushovers. However, have you stopped to think about the whole spectrum of color.
Blue is not the wavelength with the highest frequency in the visible spectrum (think back to ‘Roy G Biv’). Indigo and Violet are on the spectrum after Blue, they have even smaller, more frequent wavelengths than blue. So why is it that the sky is not Indigo? Or Violet, the shortest of all wavelengths, shouldn’t Violet be the wavelength to scatter the most?
One reason why the sky isn’t Violet is that Violet gets scattered by the higher atmosphere and is already absorbed and scattered so high above us that we don’t really see its hue in the sky. Another reason is our eyes are actually LESS sensitive to the color Violet and MORE sensitive to the color BLUE! Yes, that is right, our eyes actually pick up on the color blue easier and more readily than Violet. Let’s ‘take a look’ at the eye and see why this is so.
Our retina (which is like the film for our eyes’ camera) has rods and cones in it. The cones are the color detectors’ and there are three types, red, green and blue. When these color detectors are stimulated in different proportions, a color is formed in our mind’s eye. Since the sky has the least amount of red light scatter (remember, red is the long and strong wavelength, undeterred by particles in the atmosphere), the red cones are stimulated only a small amount when we look at the sky. The green color detectors are stimulated a little more than the red and the blue color detectors are stimulated the most when looking at the sky (because blue scatters best). The colors of Indigo and Violet actually have a slight reddish tinge to them so the eye has its red color detecting cones stimulated again, but a little more than before this time. The net result is that the red and the green color detectors in our eyes are stimulated by about the same amount when we look at the sky, while the blue color detectors are stimulated much, much more. So BLUE WINS and again, we see the sky as being the color blue, not Violet. (Sorry, Violet.)
Ok, our last quick factoid about the sky has to do with sunsets. Again, we will go back to concepts we talked about earlier in this article, one of which is how Roy G Biv demonstrates for us how ‘long and strong’ red wavelengths of light are and how small and easy-to-push-around blue wavelengths are.
The light from the setting sun has to travel a far distance along the horizon to meet your eye. That means its white light is traveling through the lower atmosphere, the air closest to earth. The lower atmosphere can be filled with large solid or vapor particles in the air such as pollutants, vapors, dust, ash, pollen and even salt from the sea. On a clear day, the sky and the setting sun will appear yellow. However, when there are solid or vapor particles in the air, these particles are big and they can sort of ‘block’ the smaller wavelengths of light before the light travels the distance to your eyes. Sometimes only the “longer and stronger” wavelengths of visible light can make it through the obstacle course of the lower atmosphere to reach your eyes. That is why our eyes see the sunset in wonderful shades of red, orange, yellow and maybe even pink.
Isn't science grand?
(The original version of this article appeared here.)
(drawings and sunset photograph all by C. Murphy; do not duplicate)