As curious children, we used to ask questions about everything: How does that work? Why does this do that? Maybe as a kid, you asked why the sky was blue; I know I did. One person might’ve said that it’s because the sky reflects the blue of the ocean (which it doesn’t). Another might’ve given a half-answer about how it has “something to do with light or the atmosphere probably.” So why is the sky blue?
About Light
In the electromagnetic spectrum of light, we only see a small segment. This visible light spectrum is a range of all the wavelengths we are able to see. This range of wavelengths consists of what we perceive as color. And yet, color isn’t an entity on its own. Isaac Newton proved with his prism experiment that colors are just different pieces of light scattered by a medium. Hence, the white light you shine into the prism comes out as scatterings of different wavelengths, causing you to see a rainbow.
Scattering
The atmosphere serves as a different type of medium. The air molecules in the atmosphere scatter the incoming light from the sun. Smaller wavelengths of light are scattered more, and we then see the light as that color. Out of the colors on the visible spectrum, violet has the smallest wavelength, which begs the question of why the sky is blue and not violet. Blue is the obvious second candidate, but on the basis of wavelengths alone, it would make the most sense if the sky were violet.
However, there are a few things we are not taking into account. First, the source of light plays a part in the way light is scattered, and our sun doesn’t emit perfectly proportional white light. Instead, there is a higher concentration of blue wavelengths represented in the light. When light comes through our atmosphere, blue wavelengths become scattered the most and the sky appears blue. Additionally, the cones in our eyes are more receptive to blue light. So even if the sky is actually more violet than blue, our eyes see the blue more predominantly.
Sunrises and Sunsets
Except, the sky isn’t always blue. As the sun rises and sets above and below the horizon, the sky appears to change colors, looking more red, orange, pink, etc. When we see the sun at the horizon, it is farther away from us than when it is high in the sky. The farther away the sun is, the more atmosphere the light must pass through in order to reach us. In this longer distance, blue gets scattered to the point where we don’t see it as much, instead leaving the longer wavelengths of red and orange that do not scatter over an extended distance.
What about the oceans?
If the sky isn’t reflecting the blue ocean, is the blue ocean reflecting the sky? While a common misconception, the ocean’s color isn’t related to the color of the sky. If it did, how would you explain water that appears more green than it does blue? Instead, bodies of water are the colors they are because of how various wavelengths interact with other substances. Water is better at absorbing longer wavelengths (reds, oranges, and yellows) and reflecting shorter ones, such as the blues that we tend to see in the water. The green-ish colors are not a result of this same absorption/reflection, but is actually because of the organisms and sediments found in that body of water.
The sky is blue because of the scattering of the sun’s light, the concentration of blue wavelengths in that light and the way our eyes register color. That’s not the kind of answer we would’ve understood as children, but we’re not children anymore. And given that adulthood supposedly gives answers to some of life’s big questions, here’s how you know you’ve grown up: you’re no longer the one asking why the sky is blue – you’re the one answering it.