No really, seriously, why is the sky blue?
Simple questions don’t always have the most straightforward of answers. I’m sure, at some point, everyone has heard a very simplified explanation describing the why the sky appears blue to us on a bright, sunny day. There are a few common misconceptions that go around such as: our blue sky “is just reflecting off the ocean”, or even “it’s all just dust particles”. While those answers aren’t fully correct, they do contain hints of the truth. In this article we’ll break down some information, in small parts, to help you fully understand why the sky is blue.
What is light?
In the research realm, light at as “measurable object” stems from the classical theory of electromagnetism. Although for our purposes, we can view light as discrete packages of energy called photons. These photons are what one refers to when they talk about the speed of light, as in the maximum speed the photons can travel unhindered. A source of light, most noticeable of which is the sun, typically appears white in color to the human eye (Disclaimer: please do not look directly at the sun!). This white light beams down and warms (since photons are packages of energy, heat is a byproduct) one half of the earths hemisphere as it rotates. This white light is scattered by multiple “things” such as our atmosphere, oceans, buildings, lakes, trees, and… well literally everything! Everything that inhabits the earth’s crust either reflects the white light like a mirror, scatters the light, or can even bend the light like in the prism below.
Looking at this prism we can start to see something a bit more familiar: a popular band’s logo that you might recognize on an old t-shirt/poster… I’m joking!… we can see colors! All the colors of the rainbow are indeed contained inside what we perceive as white light. A prism is what we call the object that bend the light into different colors. These colors are made up of photons travelling at “restricted speeds”, which is different than when they all travel at the same speed to make the light appear white. Each individual color travels through the prism in a different way, so when it comes out the other side the colors are spread out showing a rainbow. Take the picture below for example.
As you can see, different colors have varying numbers of wave peaks in the same duration of time compared to others. This visible spectrum of color can be be seen by our eyes since our human visual system allows for us to see wavelengths (wave peaks/time) on the scale of 400-700 BILLIONTHS of a meter (nanometers a second). We can also see how visible light compares to other “wavelengths” we encounter everyday (below), which encompass what we call the Electromagnetic Spectrum. This spectrum displays the sources of electromagnetism based on how fast their wavelength is.
Have you ever seen a spray of water from a hose, or splashes in a pool, and the mist causes a small rainbow? Well, the same physical principles are happening here! The sunlight is being bent through and around the water molecules, causing a disruption in how fast the light travels and our eyes can account for these changes to allow for us to see thr different colors that make up the rainbow.
Human visual system
The human visual system is restricted to seeing colors in the visible spectrum. This is due to the make-up of specialized cells in our eyes that respond to light, called receptors. In our eyes, we have two kinds of these light receptors. One we call rods for black and white vision (dark) and the other cones for our color vision (either red, green, or blue). These receptor-cells respond to their own specialized color wavelength of light and transmit that information down your optic nerve. The information then travels to a few other brain regions before finally being projected back to the occipital lobe at the very back of your brain. All of this information-sharing happens extremely fast and allows for your perception of all the colors and shades of color you see in the world. This also allows our perception to be rapidly updated when the environment we are looking at changes. Other animals, such as hummingbirds, can see and reliably distinguish between a multitude of ultraviolet ranges of light, such as ultraviolet-red, -green, -blue, -yellow, -purple. While it’s not entirely known what use this is to the hummingbirds, can you imagine what they see looking upon a field full of thousands of flowers?!
Putting it all together
All of that knowledge of light’s properties and how we see, is needed to understand why we see a blue sky. Sunlight scatters everywhere and while a small portion of the un-scattered white light can still reach us, the blue light is actually scattering and expanding faster. Remember from the picture above that blue wavelengths have the most wave peaks per time. The abundance of this wavelength makes the sky appear bluer.
Although, how do you explain why you can see lighter colors, and even white, when looking at the horizon or a beautiful multicolored sunset?
Let’s get into it!
The lighter blues and colors we see when looking closer to the horizon are due to even more scattering of the blue light. The blue light that reaches our eyes from that far way is actually scattered many times more by gases and air molecules than the light that is closer to us. It will also compete with all the scattering light that takes place on the surface of the Earth. This helps blend all the different color wavelengths together, which causes our eyes to then perceive the sky near the horizon as more of a white or a lighter blue color.
You can maybe guess where we’re going with sunsets next. As the sun gets lower in the sky, the distance between you and the sun increases. So, blue light has more time to scatter, which allows for the beautiful warm colors such as reds, pinks, and oranges (that we see in our multicolored sunsets) to break through and be perceived by our eyes and brain! It’s the extra scattering of the usually dominating blue light that allows us to better see the lower wavelength colors shine through.
To encompass all of this information into one sentence would prove to be difficult, but that doesn’t stop us from trying. So next time someone asks you why the sky is blue, keep your answer short and factual and maybe say something along the lines of…
The sky generally looks blue since the faster-travelling photons are scattered more readily and, therefore, more abundantly in the environment, our visual system perceives this phenomenon as the color blue.
References:
Mary Caswell Stoddard, Harold N. Eyster, Benedict G. Hogan, Dylan H. Morris, Edward R. Soucy and David W. Inouye, appears June 15 in the Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.1919377117). Their research was supported by Princeton University, the Princeton Environmental Institute, a Sloan Research Fellowship, and a Packard Fellowship for Science and Engineering.
HowStuffWorks.com Contributors "What is a wavelength?" 26 July 2011. HowStuffWorks.com. <https://science.howstuffworks.com/what-is-wavelength.htm> 10 April 2022