Why Is the Sky Blue? Hint: It's Probably Not for the Reason You Think
The question of why the sky is blue is usually asked by two-year-olds learning their colours for the first time, and secondarily pondered by the poor, sleep-deprived parents who have to answer the two-year-olds’ questions. Most people think that they have a general idea as to what causes the sky’s blue colour: "Uh, it’s because of ozone or something, isn’t it?" But the seemingly simple question actually has a more complex answer than most believe. Hint: It has absolutely nothing to do with ozone.
What Is Ozone?
Ozone, or O3, is a molecule consisting of 3 oxygen atoms bonded together. Most of the gas (about 90%) is found in the stratosphere, which begins at between 10 and 17 kilometres (6 and 10 miles) above the earth’s surface and stretches up to 50 kilometres (30 miles). It has distinctive blue colour, which is why so many people name it as the chief cause of the sky’s overall blueness. This is, in fact, incorrect. Instead, the blueness of the sky is caused chiefly by two other gases that are much more abundant in the earth’s atmosphere, oxygen (O2) and nitrogen (N2). Both of these molecules are much smaller than ozone, which is a key fact that you’ll need to remember for later on.
Why is the Sky Blue?
O2 and N2 are the two most abundant molecules in the air we breathe, and anyone can tell you that air doesn’t look blue. The sky, however, clearly is blue. How does that make any sense? Because of the nature of light itself. Most people are aware that the light we see shining down from the sun is actually made of seven different colours which, when combined, appear white. We know this because of objects like prisms, which refract the light and split it into its constituent colours. This is the basic principle behind how rainbows are formed.
What many people don’t know is that different colours of light have different wavelengths. Pictured below is a representation of the electromagnetic spectrum. The further to the left the colour, the shorter the wavelength. Because red light has the longest wavelength it’s much less likely to strike the tiny molecules of oxygen and nitrogen than the shorter wavelengths and instead passes through the atmosphere unhindered. Blue light, however, is far more likely to strike the gas molecules and scatter. This phenomenon is known as Rayleigh scattering.
What Is Rayleigh Scattering?
'Rayleigh' was actually named John William Strutt. In 1871, he published a mathematical proof detailing how blue wavelengths of light are scattered sixteen times more often in earth’s atmosphere than red wavelengths. His principle is called Rayleigh scattering because his official title was the third Baron Rayleigh, John William Strutt. A bit of a mouthful, if you ask me.
If Red Light Passes More Easily Through the Atmosphere, Shouldn't the Sky be Red?
Nope. Though it may seem counter-intuitive, it’s the wavelengths that scatter, or are absorbed by a certain object, that the human eye detects rather than the ones that pass through. The red, green and yellow wavelengths combine into what we know as sunlight. As such, the fact that blue light is sixteen times more likely to scatter than red light means that we see sixteen times more blue in the sky than we see red.
But Purple Light has an Even Shorter Wavelength Than Blue Light. Why Isn’t the Sky Purple?
As fantastic as it would be to wake up one morning and see that the sky had turned purple, unfortunately we’re lowly human beings and our eyes are limited. The middle colours in the light spectrum are much more easily detected by human eyes that the colours on the ends. This means that though purple light is scattered more than blue light we don’t see a purple sky because our eyes are more adept at seeing blue.
Fun fact: Bees and butterflies have a field of vision further towards the purple end of the light spectrum and are therefore most likely see the sky as purple! Lucky!
But What Actually Is Blue? Can Colour Be Measured From a Purely Objective Standpoint, or Is a Subjective Viewpoint Also Necessary to Get the Full Experience?
Oh boy. I suggest you hit the philosophy books and figure out the answer to that one yourself because I’m stumped. I’m sure there are some in-depth and deeply nuanced answers out there for you.
Did you learn something from this article?
The sky isn’t blue because of ozone, but because of smaller gaseous particles which scatter short wavelengths of light while allowing longer ones to pass through. Purple is scattered most strongly, but our eyes detect blue more readily, which is why we see an azure dome arching over us every day instead of a lilac one. If your two-year-old asks you this question, though, it’s probably better to go with something along the lines of "because it just is" and wait until they’re a little older to give them the full explanation.
- Gibbs, P. (2018). Why is the sky Blue?. [online] Math.ucr.edu. Available at: http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html [Accessed 31 Dec. 2018].
- Spaceplace.nasa.gov. (2018). Why is the sky blue? :: NASA Space Place. [online] Available at: https://spaceplace.nasa.gov/blue-sky/en/ [Accessed 1 Jan. 2018
- YouTube. (2018). Why Is The Sky Blue?. [online] Available at: https://www.youtube.com/watch?v=x0996ts017U [Accessed 3 Jan. 2018].
- Physics.org. (2018). Why is the sky blue?| Explore | physics.org. [online] Available at: http://www.physics.org/article-questions.asp?id=108 [Accessed 1 Jan. 2018].
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© 2017 K S Lane