There are many things in our life that we accept and take for granted. After all, our brain only has enough time and energy to catch up on what my friend ate for lunch, what Trump tweeted from his pot, what YouTube & TikTok recommends for my dopamine fix today…you get the point. We have a life.
We have better things to do than sit around pondering about questions like “Why is the sky blue”, “How do magnets work”, “What are time zones”, “What is gravity”. But exploring these questions makes the “(we have a) life” on Earth so fascinating. After all, scientific thinking is the reason you’re able to read this on a digital device halfway across the world from where I’m writing this now.
As Richard Feynman puts it, it is so much fun to imagine!
So why is the sky blue?
Trick question. The sky is not blue! Wait, what?
Our human eye can only “see” colors in the visible light spectrum. The sky is just transparent air.
So how do we experience color?
Sunlight looks white, but it is made up of all colors of the rainbow (infrared, visible light and ultraviolet). Light energy travels in waves. In the electromagnetic spectrum, blue waves are shorter (and higher frequency) than red. All light travels in a straight line until it gets reflected or refracted or scattered. This is when we begin to experience color.
Alright, so what makes the sky “look” blue during the day?
Sunlight falls on the Earth’s atmosphere and most of it passes through, allowing us to see things on our planet. A small part of this light gets scattered in all directions by the gases and particles in the air (a phenomenon known as Rayleigh Scattering). Oxygen and Nitrogen and other tiny molecules in the air, closer to the size of blue light’s short wavelength, scatters blue more than other colors. And so during day time, vast majority of the sky appears blue to us. By the way, ultraviolet gets scattered even more than blue, but it falls outside the visible light spectrum for our human eye to pick up.
But what about that lovely red and orange hue at sunset, the starry black night, and the rainbow?
You know the first principles now. The answer lies in what scatters the sunlight and which colors dance in the sky into our eyes. Multiple factors determine this, like
- the angle that sunlight falls at (based on earth’s rotation, revolution, and tilt)
- thickness of the atmosphere
- the dust and water droplets (clouds) and other particles in the air
Sunlight passes through more or less of the atmosphere and gets scattered and re-scattered to different degrees. This allows blue or red or orange or yellow or no colors to pass through straight to our eyes. What you see at any moment in time is the result of what gets scattered and what gets filtered through.
Go for a walk today evening and watch the marvelous sunset unfold.
Why is the sky the color that you see? Did you see other colors at the same time in another city or country? Do you think you’d see a similar sky from another planet? Why does the sun look yellow?
It is fun to imagine!
Reading: NASA on Sunsets and Atmosphere