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  • 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, theyre 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

  • individuallyROYGBIV.

  • 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 thebowin 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 youll 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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To make a rainbow, you need three ingredients: sunlight, water, and you.

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