Eli5 why rainbows appears in a curved shape? Why isn’t it just a random streak in the sky? What causes their signature curved look?
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The optical resistance (called index of refraction) is what bends light when it enters different, clear-ish substances like glass or water. The amount that light bends depends on both the index of refraction and of the wavelength of the light. Higher energy light tends to interact with the molecules more, so they get bent a bit more too. That’s what causes the light to be separated into the various colors.
What causes the arc is that there is a specific angle that can bend the light just the right amount for you to see it. If a water droplet in the air is too far to the right or left, it will not refract the light back to you. However, there are billions or trillions of water droplets so there are pretty much guaranteed to be some at the right angle. That angle can be to the left, to the right, up, down, diagonal, or anything in between. A rainbow actually makes a full circle, but half of it is usually below the horizon. If you see a rainbow in a plane, you probably will see the whole thing
Here’s a [rough diagram](https://imgur.com/a/aEtUPqd) of how this happens left to right, but this picture can be rotated any direction and there will always be a symmetric piece of the rainbow on both left and right sides. A shape that is perfectly symmetric no matter what angle you look at it is a circle.
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