Why Isn't the Sky Purple?

What makes the sky blue, and how our biological design is so beautifully crafted as to see the majesty in everything

“He covers the skies with clouds; He prepares rain for the earth; He makes grass grow on the hills.” ~ Psalm 147:8 (ESV translation)

Photo by Cristofer Maximilian on Unsplash

Why is the sky blue?

I’m sure you’ve either asked this question yourself or had a curious little kid ask you at some point. It’s one of the classic childhood mysteries.

After all, the sky could’ve been any color, right? So… why blue?

Let’s start with the basics: how we even see color in the first place.

We see things because light interacts with the world around us and reflects back into our eyes. Different materials interact with light waves in different ways—absorbing some wavelengths and reflecting others. We see these different wavelengths as different colors, and it’s the wavelengths that get bounced back toward our eyes that are the ones we see.

Most of the light we see—from the sun and even from many artificial sources—is called white light. But weirdly enough, white light isn’t just one color—it’s actually made up of all the colors, combined. That includes colors we can see and others we can’t. To help make sense of all this, scientists use something called the electromagnetic spectrum, which sorts all these “colors” by the size (wavelength) of their waves.

Light is a wave. (It’s also a particle—technically, it’s both at the same time! That’s one of the key ideas in quantum mechanics, but don’t worry—we’re staying in the everyday world for now, so we won’t dive into that today.)

Waves, no matter the type—whether sound, water, or light—have two key characteristics: wavelength and frequency.

Wavelength is the distance between two peaks of a wave. Frequency tells us how often those peaks pass by a given point. Short wavelengths = high frequency. Long wavelengths = low frequency.

With sound, these different wavelengths produce different tones, like the different strings on a guitar. Liquids are pretty simple, you can physically observe these characteristics in simply seeing how big the waves are and how fast they’re traveling.

For light, however, different wavelengths correspond to different colors.

On the far end of the spectrum, we’ve got super short waves like gamma rays, X-rays, and ultraviolet (UV) light. On the other end, we’ve got long waves like infrared, microwaves, and radio waves. Right smack in the middle is this small, magical sliver called the visible light spectrum—the only part we humans can actually see. It ranges from violet (shortest visible wavelength) to red (longest). It’s from this tiny band of light that we see the full rainbow of colors!

When light enters our atmosphere, it interacts with the molecules in the air in a process called Rayleigh scattering.

Rayleigh scattering is just a fancy term for what happens when light hits tiny particles, like the gas molecules in our atmosphere, and gets scattered in different directions. The shorter the wavelength, the more it gets scattered. The longer, the less so.

Blue light, being shorter than red or yellow, gets scattered far more across the sky. It’s that abundance of scattered blue light that is what paints the heavens in such a soft, vivid hue.

But… that doesn’t explain everything.

I just said that shorter wavelengths get scattered more. Blue light is short, right, but it’s not actually the shortest. Purple is!

So… shouldn’t the sky be purple?

Turns out, the answer actually lies more in our own biology than in the physics of light.

I explained the physics behind why we see colors, but the physiology of our own bodies actually plays a pretty important role in that as well.

The retinas of our eyes have three types of special cells called cones, each of which is sensitive to different wavelengths of light: one for red, one for green, and one for blue. These are the primary colors of light, and every other color we see is some mix of these three. The exact “color” we perceive depends on the relative intensity of each wavelength that our eyes detect.

If you’ve ever played around with mixing paint or some other art medium, you’re probably familiar with the idea that different amounts of these three colors gets you different shades or even brand new colors.

It’s the same idea here—just with light!

Over here in physics world, we call that intensity. Simply put, how much of each color you’re getting.

If something scatters more red and green than blue, you’ll probably see some shade of yellow. Scatter more green and blue than red, and you’ll get a teal or cyan! All different colors, of all different shades, are created from these three colors.

That’s what those three little “cones” in our eyes were built to detect.

So even though violet and purple light are scattered too, our eyes simply aren’t as sensitive to those wavelengths. They weren’t designed that way. The “blue” cones in our eyes are actually more responsive to blue light than to violet. On top of that, sunlight actually contains more blue light than violet to begin with, so blue wins out visually.

We see it as blue because that’s what our eyes were made to see.

And honestly… I think that’s kind of beautiful.

It’s not just the universe that was carefully designed—it’s us, too. Designed not just to survive, but to see and feel and be moved by the world around us.

Psychologically, the color “light blue” evokes feelings of calm, peace, and serenity. I don’t know about you, but few things are more inspiring or peaceful to me than gazing up at a bright blue sky. Fluffy white clouds drift across that vast expanse, perfectly complementing the green leaves and brown trees of the earth below. The color of the sky harmonizes so well with the rest of the natural world—it’s almost like it was meant to.

Crazy!

Blue and green are even next-door neighbors on the color wheel. Blue simply pairs more naturally with the world around us than purple would. It’s better suited to inspire awe, to bring peace.

And maybe that’s exactly why God made it that way.

To reflect His glory.

To draw us toward Him in wonder.

To give us something beautiful to lift our eyes to.

And I think that’s just… amazing.

—See you in the stars—