“Why is the sky blue”?
It sounds like a question that’s as old as time, but it isn’t that old at all.
When the Greek poet Homer wrote the Odyssey and the Iliad, he never once used a word for
“blue”.
From Greece to Africa to China, blue turns out to be one of the last colors that languages
figure out a word for.
For a long time, many people thought that the sky was blue for the same reason the ocean
is blue: Air, like water, is only very faintly colored blue.
So though a small amount appears to be colorless, an ocean, lake, or the sky naturally looks
deep blue.
Of course, that’s COMPLETELY wrong, and the sky isn’t even always blue, right?
At the beginning and end of the day, it looks red or orange.
So now we have two questions: Why is the sky blue, and also why is the sky red?
It turns out, the sky is red BECAUSE the sky is blue.
Let’s me show you why that is.
It’s 3 PM and you’re on a beach in sunny California, staring up at a blue sky.
At 2:52 PM, just eight minutes before, a beam of light left the sun traveling to Earth at
just under 300 million meters per second, carrying every wavelength from x-rays to radio
waves on their eight minute trip to Earth.
Less than a millisecond before 3 PM, high above you, that beam of light enters Earth’s
atmosphere.
On its way through the atmosphere, that sunbeam will be trimmed down to just a narrow range
of wavelengths.
But that light still appears white, a mixture of all wavelengths.
Here’s where it gets cool.
A physicist named John Strutt, figured out that air molecules can actually scatter light
as it travels through the atmosphere.
It turns out that how much they scatter depends on one over the 4th power of the wavelength.
That means short wavelengths like blue and violet get scattered more than long wavelengths
like red and orange.
Some of those blues and violets scatter down into your eyes.
They get mixed with some white light, and you end up seeing a color that looks a lot
like this.
But violet light has an even shorter wavelength than blue, so why isn’t the sky violet?
That’s because the color receptors in your eyes have evolved to be more sensitive to
blue than violet.
By now it’s 3:01, and while you’re busy thinking about purple skies, your phone rings.
It’s your friend calling on the beach in Bermuda, watching the sun set four hours ahead
of you.
Those colors are a result of the same science that makes the sky blue.
When the sun is just above the horizon, the light in a sunrise or sunset has to travel
through more atmosphere than at any other time.
All that’s left, all that DOESN’T get scattered, are the longest wavelengths: red,
orange, yellow.
The blue that’s missing?
That’s the blue that you see, thousands of miles away.
They say: “Wish you were here!”
and in a way, you are.
You’re each looking at different pieces of the same sunbeam.