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[music]
From coloring books to record covers, rainbows are everywhere. I bet you can even remember
the colors without thinking twice.
Although in real life no one really says "indigo", although I guess ROYGBV doesn't really have
the same ring to it.
As anyone who's tried to chase one finds out, a rainbow isn't really there. You can't go
over it, and you can't get to the end of it. It's become a mythical representation of the
unattainable. In Navajo and Norwegian mythology, it's a bridge that only gods could take between
heaven and Earth.
For Christians and Buddhists, rainbows are a state of peace and forgiveness. And in some
cultures, it's a really long and colorful unicorn.
Of course, just because a rainbow isn't really there doesn't mean we can't explain how it
works. The better question is "WHY is a rainbow?" And that answer is "42" Let me explain.
A rainbow exists because of light, water and a little physics.
Let's start with sunlight. It looks white. To some that seems like the absence of color.
But thanks to Isaac Newton we know that white light is really the sum of all visible wavelengths,
from short to long and all the colors in between.
So we've got light, now we need water. On a rainy or misty day, the sky is filled with
tiny droplets. They aren't quite as small as the droplets in clouds, though, which is
why we don't have awesome looking clouds. Some of you might be saying "But Joe, I've
seen a rainbow in a cloud before! Well, that's not really a rainbow. But we'll talk about
that another time.
Those suspended liquid prisms are surface tension on its smallest scale, and they're
pulled into the shape of a sphere and each one can catch sunlight and become its own
part of the rainbow factory. Here's where we add the physics.
Sunlight starts by entering a raindrop from behind you. The light goes from one medium,
air, into another, water. And that causes it to bend slightly thanks to a process called
refraction. Different wavelengths of light bend at different angles so the white light
begins to separate. This bent sunlight then reflects off the back of the raindrop, and
refracts again on the way out. When we measure the angle between the light that went in and
the red light that comes out, the answer is 42 . . . degrees.
So how many drops does it TAKE to make a rainbow? Well, a lot. Because each color exits at a
different angle, one raindrop will send red light into your eye and another drop will
send violet light. The same thing happens with all the colors in between, each coming
from their own droplet.
And what about the shape? We call it a rainBOW for a reason. It's not a rainLINE or a rainZIGZAG.
You and your eyes are at the apex of a huge half-cone, and at the other end is the water
that makes the rainbow shape that we all know and love.
Everything that you see is because that light is refracting right to where you are. No one
else is experiencing exactly the rainbow as you.
If everyone's rainbow is unique, does your rainbow look like my rainbow? Well, Michael
from Vsauce has a great video about that, so I'll put a link in the description to let
him answer that question for you.
Could we ever see a different rainbow? We've evolved to see only a tiny fraction of a percent
of the entire electromagnetic spectrum, what about all the rest of it, from x-rays to radio
waves. What if we could see a rainbow like the mantis shrimp, maybe we could see a rainbow
that stretches across the sky!
Spoiler alert, maybe NOT.
While it's true that we only see a sliver of the spectrum, you can't make a rainbow
out of anything that doesn't make it into Earth's atmosphere. Take the sun: It actually
emits almost half of its radiation right around the visible range, which is exactly why we
evolved to see that range. The rest of the sunlight is mostly infrared and just a tiny
bit of UV. If we could see a rainbow made of all of that, it would only be about twice
the width of the one we're used to. Unfortunately, if we could see infrared, we'd be blinded
by the infrared radiation given off by all the warm things around is, including the Earth
itself.
Cranking up the sun wouldn't work either because most of the electromagnetic spectrum is filtered
out before it reaches Earth's surface.
What about all those radio waves, they're traveling freely through the atmosphere. Picture
all those bands way beyond the red side of our rainbow. Sadly, having radio antennas
for eyes would also be a blinding experience, because the air is filled with the signals
from our wi-fi, mobile phones, and well, radio.
I guess we'll have to be happy with the rainbow we have, because it's pretty much the best
rainbow we can make.
The next time you see a rainbow, remember that even though there's no pot of gold at
the end, no one else can see exactly same rainbow you can. That sounds like treasure
to me.
Stay curious, and thanks for watching.