To make a rainbow, you need three ingredients: sunlight, water, and you.
Sunlight, as you probably know, consists of all of the colors in the visible spectrum,
as well as a bunch of wavelengths of light that we can’t see.
And when light travels from one medium -- like air -- to another, denser medium -- like water
-- it slows down, and exits the new medium at a different angle than it entered.
This is called refraction.
In the case of water, light can enter a droplet, bounce off its inner surface, like a mirror,
and then exit at sharp angle.
And because each color has a different wavelength, they’re each slowed to a different degree
and refracted at a different angle.
So red light will exit the drop at 42 degrees from the angle at which the sunlight entered.
But blue light, near the opposite end of the spectrum, will exit at 40 degrees, with the
other colors somewhere in between.
The combined effect of this differently refracted light scatters the colors so you can see them
individually — ROYGBIV.
And this is also where you enter the equation, because the conditions have to be just right.
Rainbows only happen when the sunlight is coming from behind you and is low in the sky.
As the sunlight shines into a curtain of raindrops in the air in front of you, only one color
from each droplet will refract at the exact angle necessary to directly reach your eye.
So in one part of the sky, all the raindrops will bounce red light into your eye.
All the other colors from those particular droplets will scatter either too high, too
low, or too far to either side for you to see them.
But just a few degrees away in the sky, the blue light bouncing out of those raindrops
will be the ones to reach you.
With all those droplets refracting only a certain wavelength of light that hits your
eye, together they create the illusion of a rainbow.
So what creates the “bow” in a rainbow?
Well, rainbows actually form in a full circle in front of you, at an angle of 40 to 42 degrees
from your line of vision.
This means you’ll always be at the center of any rainbow you see!
Which is kind of a nice thought.
But it also means that the Earth is going to block the lower half of the rainbow, so
you typically only see the upper arc.
However, some extremely lucky skydivers, pilots, and mountaineers have gotten high enough above
the horizon precisely in the right conditions to see a full circular rainbow.
It DOES exist.
But I am perfectly happy just seeing half of the rainbow and staying right here on the
ground, thank you very much.
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