Why is the sky blue?

Episode #1 of the course “Science questions everyone should know how to answer”

Every day, billions of people look up at the sky and see blue. We all know the sky to be blue, but here’s the thing—it’s not. The sky is made up of air, and air is translucent, or clear. The air in front of your face right now has no color, does it? So then why does the air in the sky have color, specifically a blue color?

We see the sky as blue because of three things:

1. The sun

2. Particles in the atmosphere

3. And the fact that we’re humans

Let’s start with #2, the particles. In our atmosphere, there are billions upon billions of tiny molecules of oxygen and nitrogen. These tiny molecules are floating all around us. Next, onto the sun, which emits light rays. Light exists in a large spectrum, only a small section of which is visible to humans. The spectrum that is visible to humans is made up of the colors or the rainbow, from red to blue and violet. This is the key: the waves of blue light are shorter and more frequent than the waves of red light. Because the blue waves are shorter, they are more likely to collide with those tiny oxygen and nitrogen particles, causing that light to scatter. While the rest of the wavelengths pass through the atmosphere, lighting up the ground and everything we see, the blue waves get stuck in the atmosphere, thus creating the blue color.

But what does being human have to do with it? Well, if you’re especially clever, you might have wondered why the sky isn’t violet. In the spectrum of light we see—the colors of a rainbow—red wavelengths are the largest, so that would make violet the shortest, not blue. This is true, but human eyes do not perceive violet very well, so even though there are more violet wavelengths being caught in the sky than blue, humans just don’t see them. Other animals do, though, which means the sky probably looks totally different to them!

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