Why is the daytime sky blue?

Most of us know for a fact that the daytime sky is usually blue on a sunny day but few of us stop to ask why. We know the sun isn’t blue so why does the sky turn that color when the sun rises? Why isn’t the daytime sky orange, or yellow? Well, if we were to venture a collective guess, we could reason that the “blueness” of the daytime sky is probably due to the risen sun. This guess makes sense because when the sun completely sets, the sky turns black except for maybe the moon and little specks of light, which are actually other stars from distant solar systems.

According to our current level of scientific understanding, we humans perceive the daytime sky as blue in color for two primary reasons. The first reason is because of the way our atmosphere interacts with light from the sun. The second reason is because of the way the human eye detects color. Let’s look at these two points from a deeper perspective to see if we can really grok why the daytime sky is so often colored blue.
 
 

How does our atmosphere interact with light from the sun?

In the realm of physics, visible light is an electromagnetic wave. An electromagnetic wave is a form of energy that races through space at 3 X 10⁸m/s, and possesses both electric and magnetic properties… hence the name “electromagnetic”. There are a lot more technical definitions of electromagnetic waves, but the above should suffice for the purposes of this article. Electromagnetic waves differ from the other major category of waves – mechanical waves – mainly because they don’t need a medium through which to travel. For instance, sound is a mechanical wave… and the main reason why you can hear your iPhone when it rings is because sound can travel from its ringer to your ear using the air around you as a medium of transport. If that same iPhone rang in outer space where there is no air, you wouldn’t be able to hear it. Weird right? Yeah… I know. In contrast, light from the sun (which is an electromagnetic wave) can make it all the way to the earth even through outer space where it doesn’t have the medium of air to pass through. Visible light is just one of the many different types of waves in the electromagnetic spectrum. X-rays and radio waves are also examples of electromagnetic waves… they just exist outside the range that we humans can actually see. The reason why we can see white light and not radio waves is because the fundamental frequency of visible light is at the “goldilocks setting” that stimulates and elicits a response from the “cone” cells in our eyes. Radio waves and microwaves have different fundamental frequencies which our eye cells cannot detect as readily.

If asked, most adults would probably say that the sun emits white light. While that statement isn’t false, it doesn’t cover the full depth and complexity of reality. In truth, white light from the sun is really just a combination of a bunch of different colors that make up the visible light spectrum. Sir Isaac Newton (1643 – 1727) conclusively proved that white light is comprised of a spectrum of different colors by passing white light through a prism which caused it to break up into its constituent colors. The colors that make up the visible light spectrum are: Red, Orange, Yellow, Green, Blue, Indigo, and Violet… as a random aside, you can use the mnemonic ROYGBIV to remember them. You might be scratching your head at all this wondering what any of this has to do with why the sky is blue… completely understandable. Keep reading… we’re almost there.

Now that we have a basic understanding of the nature of white light as a collection of electromagnetic waves, we are much better placed to understand why the daytime sky is blue. Each electromagnetic wave has two very fundamental interrelated properties – frequency and wavelength. The mathematical relationship between frequency and wavelength is as follows:
 

 
In plain english, the mathematical equation above simply means that the higher the frequency of a wave, the shorter its wavelength. Conversely, lower frequency waves have longer wavelengths. Below are the fundamental wavelengths of the colors in the visible light spectrum of electromagnetic waves:
 

 

 
The blueness of our sky is in part due to a phenomenon known as Rayleigh scattering which is the selective scattering of light when it comes in contact with particles that are very small. In plain english, a light wave will be deflected fairly evenly throughout the atmosphere and towards our eyes if it hits something that is about one tenth of its wavelength in size. The earth’s atmosphere is mostly made up of really tiny oxygen (O₂) and nitrogen (N₂) molecules. When white light from the sun gets to our planet, it collides with the oxygen (O₂) and nitrogen (N₂) molecules present in our atmosphere, causing light to scatter in a reproducible way. In general, the smaller the wavelength of the wave in the visible electromagnetic spectrum, the better it gets deflected by oxygen and nitrogen molecules. As a result, the Blue, Indigo, and Violet waves in the visible spectrum are most significantly scattered by our atmosphere because they have shorter wavelengths. OK so assuming you buy all the physics we just talked about, you’re probably asking yourself why the sky isn’t violet. After all, the explanation did say that the shorter the wavelength of the electromagnetic wave, the better it gets scattered by the gases in our atmosphere. As such, shouldn’t the sky be violet since the violet light wave has the shortest wavelength? This line of thinking is absolutely correct in principle, but clearly doesn’t jive with our everyday reality. The question is why? Well, the source of the discrepancy literally lies in our human eyes.
 
 

What part do our human eyes play in all this?

The human eye is truly a marvel of biological engineering. It is one of our most vital instruments for detecting and making sense of the outside world. Because of our eyes, we can see things at distances near and far in living color. Our eyes have a number of specialized cells in them one of which is called a cone or a photoreceptor cell. In plain english, the word “photoreceptor” means receiver of light. Cone cells are responsible for detecting color in a way that is probably counterintuitive to what you may think. For instance, when you see a red ray of light, the cone cells in your eyes detect that as an electromagnetic wave with a wavelength of ~650 nanometers. This signal is then relayed to your brain which makes the connection and prompts you to blurt out… “Oh… that’s a red beam of light”. There are three types of cone cells in your eyes. One of them is designed to detect blue light, another is designed to detect green light, and still another is designed to detect red light. Of the three light waves that are scattered most by the oxygen and nitrogen molecules in our atmosphere (violet, indigo and blue light), our photoreceptor cells are most sensitive to blue light. This is the other big part of the reason why we humans see the sky as blue.

It is no accident that green, blue, and red occur very often in nature… probably the evolutionary reason why our eyes evolved to pick up those colors most clearly. After all, green probably meant food in the form of fresh vegetation, red probably meant blood/danger, and blue probably meant water to the primitive brains of our ancestors… all very important things to detect in order to survive. Curiously enough, other animals that have different photoreceptor cells probably experience the sky as a different color. Science can indeed be weirdly fascinating. From all of us here at chubaoyolu.org. Take care of yourselves, and each other.
 
 
Without Wax
Oyolu B.C. Ph.D.
chubaoyolu.org
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