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?
In: 3
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.
Think of it this way…
Every single raindrop being lit by the sun throws back a very feint circular rainbow at a certain angle to the incoming sunshine. What you see is the accumulated light being thrown back from the particular raindrops that are aligned just right for your eye. This would be a complete circle, except usually the ground blocks around half of it.
This means that if you are up in the sky at the time, you can see an entire circular rainbow. It also means that every viewer sees a slightly different one. It also means that since it is purely an optical illusion, rainbows don’t have a physical end, so sadly there is no pot of gold…
Rainbows appear in a curved shape because of the way sunlight is refracted by water droplets. The sunlight is bent as it passes through the water droplets, and this causes the different colors to be separated out. The result is a beautiful, curved rainbow.
A circle is defined as the shape for which every point is the same distance from the center.
All rainbows take the shape of a circle because they occur due to light bending at a particular angle from the light path. Because the angle depends on the distance from the center, every point with the right refraction angle will be at the same distance from the center of the light path which will then produce a visible circle, the rainbow. It just happens to be that often this center of the light path is low enough that you can only see about half the circle.
All these answers have not quite pointed at the reason. It is because the sun is a point source of light, And your eye is a point of termination. So the arc is the intersection of two cones with vertices at the sun and at your eye.
Latest Answers