If you have kids, there’s a 100% chance they will, at some point, ask you why the sky is blue.
If you don’t have kids, hey. There’s a good chance you asked this question at some point, and even if someone gave you the correct answer, you’ve probably forgotten what it was.
And sure, you could just go in the other room and ask Alexa, but what’s the fun in that?
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It was Isaac Newton who came up with the theory that explains how we see light and color – ie. that light generates color. He used a prism to illustrate the fact that even light that appears “pure white” actually contains many different colors, and his investigations are essential to understanding why we see the sky as blue.
The lenses of our eyes essentially act as prisms, working to refract light and change it so that we see different colors.
Refraction is the way a wave changes when it passes through one medium into another – through a lens or a prism, in this case. So, when we look up at the sky, we’re seeing “white” light that has been refracted by the lens of our eye, but that has also been scattered by particles and molecules in the air, which is the fundamental reason it looks blue to us (most days).
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So, how do molecules scatter light? 19th century physicists John Tyndall and John William Strutt were the first to suspect dust and humidity were behind it, but it took a better understanding from Einstein, who realized the atmosphere itself contained molecules to really clear it all up.
But why is it blue, and not green or red or purple?
It has to do with the length of light waves. Blue is relatively short with a tight fold, and as light passes through the atmosphere, it is distorted into that short frequency that the color receptors in our eyes (cones) receive as blue.
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The same principles allow for the variations we glimpse during sunset and sunrise – when the light is not as direct, the molecules in the atmosphere scatter light at different frequencies.
In a nutshell, you can tell your kids (or any curious person) this: “White” light is scattered by molecules in the sky, broken into different waves and frequencies that, to the cones and receptors in our eyes, are perceived as blue.
Or you could just show them the scene from Pollyanna with the creepy old guy and the house full of prisms.
