Blue Sky Thinking · 10:20pm Nov 29th, 2014
It was a beautiful sunny day. The sky above Ponyville was a perfect photogenic image of fluffy white clouds against a blue background, with a touch of brighter coloring where a pegasus tail or two was visible hanging out of a cloud.
Rainbow Dash stretched out her wings and legs, enjoying the feeling of smug satisfaction which comes when resting after a vigorous afternoon of showing off your top stunts to your fans. Next to her Scootaloo lay in the soft white fluff, enjoying the feeling of serene happiness which comes when you're lying next to the most awesome pony in the world who has taken you under her wing.
“The wispy ones are the cirrus clouds,” said Rainbow, staring up into the sky. “They're real high, and can move fast. It's fun to chase them. Then there are the cumulonimbus clouds. They're good to hide in, and make lightning—perfect for playing pranks. And there are stratocumulus clouds, which make good race markers.”
Scootaloo listened happily while Dash preened her feathers.
“Rainbow Dash,” she said, staring upwards with a thoughtful expression. “Why is the sky blue?”
Dash stopped working on her wings and paused, unsure how to answer this.
“Because it's an awesome color for a sky,” she said eventually.
“Yes, but why?”
Dash didn't know. Why didn't they teach this stuff at flight school? Slightly annoyed, she rubbed Scootaloo's head with a hoof.
“Hey, that's an egghead question!”
“Don't you know?”
“Of course I know.” She paused and Scootaloo turned to face her waiting for the answer. “At least, I mean, I know how I can find out. You see, being awesome isn't about knowing everything—that's what eggheads do—it's about knowing enough to do what you want, and having friends who can tell you everything else if you need it. So if I want to know why the sky is blue I just have to go and ask Twilight.” She lay back in the cloud, pleased that she had managed to think up a good response.
“Can we go and ask Twilight?”
Dash lifted her head off her cloud pillow and stared into Scootaloo's bright-eyed, inquisitive face.
“You really want to know, don't you?”
“Yes.”
“Come on.”
She lifted the little filly onto her back, and jumped off the cloud.
Five minutes later they found Twilight Sparkle, busy cataloging the new books inside the library. She had just devised a new classification system in which each book was labeled by a colored alphanumerical code on the spine to identify its subject matter, language and author. This required relabeling her entire collection.
“Hey Twi,” said Rainbow. “Tell us why the sky is blue.”
Twilight grinned at the unexpected pleasure of an afternoon science test.
“It's due to the wavelength dependence of the scattering of light by the nitrogen, oxygen and other molecules in the atmosphere. Shorter wavelengths—which make up blue light—are scattered more than the longer wavelengths—which make up red light. Hence the sky looks blue. Except when we look at a sunset, when we see the red-orange light transmitted through a long distance of air.”
Scootaloo whispered to Rainbow Dash, “What does that mean?”
“Err...” Dash looked around the library hall looking for help. She caught sight of Pinkie Pie sitting on a cushion reading a comic. “Just a moment...” She flew across to her and whispered into a pink ear, “What the hay did that mean?”
“Duh,” replied Pinkie, bouncing across the room to join Twilight and Scootaloo. “You shine every color onto the sky, and the air kicks the blue all over the place and lets the red through. You just need a whole sky-full of air to see it. It also works with water and flour, milk or coffee cream. It probably also works with chalk, but I've not tried that, and cream tastes better than chalk anyway.”
“Err, what?”
To illustrate the principle Pinkie donned a chef's hat and apron, produced a large glass bowl of water from behind a bookshelf, and tipped in a bag of flour. She mixed this vigorously, sending droplets of flour and water flying around the room. (Twilight quickly cast a spell to shield her books with a protective force field.) Then throwing aside her apron and hat, she strapped a headlamp around her pink mane, jumped over to pull the curtains across the window, plunging the room into darkness. She then shone her light into the water. A bluish-white color was reflected back.
“Oh,” said Scootaloo, “now I see.”
“And from the other side...” Pinkie jumped around the bowl to the side opposite Scootaloo and crouched down to shine the light through the water so the light Scootaloo saw was the orange color of her coat. “Sunset!”
“That's not quite the same thing,” objected Twilight. “Flour particles are much bigger than air molecules, so the light is scattered by a different process. It just happens to give the same result.”
“That's good enough for me,” said Dash.
“And me,” agreed Scootaloo.
“Where did you learn that trick Pinkie?” said Dash.
“Baby-sit the Cake twins and you learn everything you need to know about flour and water.”
Vectors by HerrMyrddin and tamalesyatole.
And, in light of the discussion, Twilight dubbed her new invention the Dewdrop Decimal System.
"Regulations," Rainbow Dash replied, rolling her eyes. "Back when I was your age, they let the sky be all kinds of colors, like green or purple or this really kickin' red. It didn't always work, but it was always interesting. But then there was this big scandal at the Weather Bureau and the mare in charge of that kind of stuff had to resign. Turn around so I can get your other wing, squirt." After Scootaloo settled back down, Rainbow Dash continued, "Anyway, the mare they got to replace the old Chief must have a few sticks up her butt, because ever since then, it's just been this boring old blue every day. I actually tried to break into the warehouse once, but that thing is crazy secure."
¡Oopsy!
Got that reversed:
Since you covered the color of air, next you can cover the color of DiHydrogen MonOxide. The answer is blue. This is partially due to Rayleigh-Scattering, but water is intrinsically blue because water tends to absorb red light. Absorption and emission of light is a quantum-phenomenon. DiDeuterium MonOxide is clearer than the more blue DiProteum MonOxide due to shifts in abortion of light the more massive hydrogen causes, so the white bottom of a swimming pool full of heavy water would appear whiter than the bluish bottom of a white swimming pool full of light water
I love your Pinkie Pie she is great.
Very cool as always n_n
The really fun question to ask, when you have just enough physics knowledge to annoy physics majors, is
The answer is actually pretty neat.
2623931
Thanks
2624500
My guess at this was that what we see is the solar spectrum combined with the 1/(wavelength^4) dependence of Rayleigh scattering. The solar spectrum drops off in the violet part of the spectrum. So the light scattered by the sky is a mix of all colors, but strongly peaked around blue.
Checking with google, it seems there is more to and it involves how humans perceive color.
Thanks
2624573
Hehe, yup. This is why it's really fun to ask physics majors about this, because it actually isn't purely a physics based answer, even though it sounds like a physics question.
It's biology and how the human eye perceives color.
There are actually lots of fun little things like that, where you take something that people are familiar with, and with one, seemingly innocent, question you can turn it on its head.
Really great questions like, "Why does a mirror change an image only on a horizontal axis? Why doesn't it also flip the image vertically?"
or "A train has a solid axle, forcing the wheels to rotate at the same rate, so how can a train turn then?"
2624581
If one uses a pencil, paper, ruler and protractor, one sees that a mirror is a plane of symmetry with the reflected image equal apparent distance from the mirror as the real objects.
The tracks guide the train.
2626058
I get the feeling you just confused a lot of people if they read that.
You're missing the point of the question. As you know, when the tracks curve, the inside rail is going to traverse a shorter distance than the outside rail. So, if a train's axle is solid, forcing both wheels to rotate at the same rate, how can it turn, which would require the outer wheels to travel a longer distance than the inner wheels?
Here's a hint: the answer is related to how trains stay on the tracks.